You've now been involved for the past four series of Dragons' Den. What has been the highlight for you so far?
It would be very difficult for me to describe a highlight. It's one of those experiences that peak and trough all the time. There are days when I think I've seen some fantastic entrepreneurs and there are times when I think I've wasted a day of my life in there. But it has put me into a very challenging environment. The other dragons are extremely intelligent, smart people who do not cut you any slack and it's nice being among a group of people who make you do your best.

How many investments did you make over your first three series? Are you still involved with all of them?
Over the first three series I made six investments. I actually offered on more than that but sometimes things fall out during the due diligence process. I think my success rate is around two-thirds because I offered on around nine or 10. But I'm very pleased to say that every single one of my investments is either on track or ahead of track so it appears I have made some good decisions.

Is there one investment that stands out for you?
How I would group them is the grown-up businesses versus the punts. I've had some really exciting punts. With Sarah Lu at Yoo Do Dolls, it was a £35,000 investment and she was a very creative girl and full of energy and I was thinking "this could work". That was an unusual investment for me. Usually I want to know the figures, the background and where the market is but I'm pleased to say that what I saw in Sarah inside the Den she has not failed to deliver outside.

At the other end of the business I've got some very grown-up businesses. I've invested with Theo [Paphitis] on a green haulage company [JPM Eco Logistics] and they had three vehicles when they came into the Den. We plan to have 20 by the end of the year and we've already got 12 on the road. They were only in the Den less than a year ago. It's very difficult to compare them but those are two pretty outstanding businesses.

How has your own life changed since you first appeared on the programme in 2006?
I had a very nice life before Dragons' Den and I have a very nice life now. I have fought very hard to keep my life in balance because it wasn't missing anything before I joined Dragons' Den and, if anything, I was reticent about joining because I didn't want my life to change radically. But I have experienced things I never would have experienced. The business world keeps itself to itself but sitting in this world between business and media I meet people I would never have met.

Do you get recognised a lot more now?
Before Dragons' Den I wasn't recognised at all. Recognition was not something that either bothered me or I craved. It just happened after Dragons' Den. Now my profile through Dragons' Den means I probably can't walk anywhere without someone recognising me. It can be unnerving but most of the time it's fine.

How well do the five Dragons get on? Who do you prefer working with?
The five dragons get on very well, bearing in mind we're all competitive people and it's a very charged environment. When we're all sitting there competing for pitches then it can get very hot among us, as well as for the entrepreneurs. That doesn't mean we don't have our differences and we don't challenge each other because absolutely we do. But, very much like business, what goes on in the Den stays in the Den.

I've got more investments with Theo and I suspect that's because we have a very similar approach to business and we work very well together. We've got this well oiled machine so when we make an investment we walk out of the Den, press a button and the whole thing happens very smoothly. I've invested with everybody other than James, and that's for no reason other than that we haven't agreed on an investment together. But I would invest with any of the dragons.

What has your experience on Dragons' Den revealed about potential entrepreneurs in this country?
There have been lots of people with very good ideas and lots with very bad ones, but the great news is that there are lots of people with ideas. At least that means that people are thinking and while that might be a crazy idea one day they may come up with a good one. I'm actually very encouraged by the amount of people who get off their backsides and put themselves in a pretty difficult situation. A lot of times I also feel very depressed about some of the pitches but overall I'm very pleased by the level of true entrepreneurism in this country.

What are the biggest mistakes you see people make in the Den?
One of the biggest mistakes I see - and people will recognise this - is people not knowing their numbers. Too often I hear "I'm not the numbers person". That's not good enough because you have to have a basic understanding of your business. You don't have to be a finance director but you need to know which numbers you do have to remember and it's usually very few. I used to run my entire holiday park business on five key numbers.

When you make an investment in the Den, what are the key things that have to be right for you?
It's very difficult to pinpoint exactly what it is; it's about the right combination. There's one rule I do follow, however, I will never invest in a good person with a bad product because that encourages a person who's probably got a very good career or business ahead of them to spend too much time on the wrong product. What I will invest is a very good product, not necessarily with a bad person, but with someone who doesn't have the skills to take it forward because that's where I come in. I can probably bolster up their weaknesses. The magic is when you get that combination of a good product and someone who has got that whatever it is. And you can sense that in the Den among all the dragons. It's almost at the same moment; you get this charged electricity.

When you first came on the show, you came across as quite a formidable, no-nonsense businesswoman. Was that a fair impression?
I'm aware that I come across on Dragons' Den as formidable and tough and people have used a lot harsher words than that about me. The other dragons and my friends try to defend me and say that I'm not like that in real life but actually I am in that environment. I'm not trying to win a Miss Congeniality contest; I'm not even trying to network and make friends. In that environment I'm there to make good, sound business investments and I'm only interested in that. I want to put these people under pressure because I want to know how they behave under pressure. So in that environment it's me. But it's not the real me; nobody is like that their entire lives.

Do you think you have mellowed slightly over the past two series?
It's difficult for me to know if I've mellowed because I am what I am. I don't consider the way I'm coming across. I don't think I've changed. I hope I haven't because my job hasn't changed. Maybe I've relaxed more because in series one I was new and I didn't know how it all worked but I hope I haven't fundamentally changed.

How much of your time is spent on Dragons' Den investments?
I spend as much time as is needed on my investments and that's either me putting the time in or putting in people who have got more experience in certain areas. I believe that once I've committed to an investment I should give it the best chance. Some of these businesses need very little of my time but what they actually need is my money. Some other businesses will need a disproportionately large amount of my time but as long as I believe that it's moving forward I will do whatever is needed. If I ever think that I'm investing my time needlessly and the business isn't going anywhere then I will stop doing it.

Extract from full interview conducted by Nick Martindale for New Business magazine