TIP: Set a minimum charge for PAT testing at £50 or 20 items @ £2.50 - This is what I do and have a set downloadable file on my website for customers to read - if they don't like it then they can go elsewhere.
Remember you're in business to make a living, not be a charity case for customers!
Tel's answer above is quite informative. You need to own your cost and income structure, no-one else is going to do it for you. Work out what your costs are, how much profit do you want to make (don't forget tax, NI etc), and there's your price to the customer. If others are charging less and you're not winning jobs because of price, it's time to review costs and profit. Best approach is to do a monthly target (for costs and income) for the whole year and that'll give you an idea of whether you're on track or not as you go through the year. There's plenty of small business advice about this sort of stuff on the web.