If it is straightforward, like "I leave everything to x", I picked up a simple blank will at Woolworths for a couple of quid. You need someone to witness your signature who is not a beneficiary.
Alternatively, http://www.rememberacharity.org.uk/serve.jsp?p ageID=12 may help. (I think you can use their forms even if you do not intend to leave anything to a chaity.)
You can get a pro-forma from your local stationers (or possibly Post Office if you still have one) Some notes of guidance come with the form, but you should be able to get a book from the library explaining the ins and outs.