I used to write wills for a living. However, prior to that I'd had my own will prepared by a solicitor. It was only when i came to study the pitfalls that I realised how many mistakes the 'professional' had made!
My advice is to get hold of a copy of this book:
Amazon.co.uk User Recommendation
(It's in most libraries).
Read it carefully and draft your own will (taking into account all of the "what if?" questions you need to think about). If you're completely confident with what you've got then go ahead and get it signed and witnessed. If not, take your draft to a solicitor and ask him/her to tidy it up.
Chris