It depends how generic your life is.
You know a bit about my family - older wife, her kids, non of my own, handicapped brother I am responsible for etc. This doesn't fit the write your own method.
Things to think about:
Start simple - Mrs DrP gets everything when you die = easy, write your own
+1 = You want to leave some money to a relative/friend/me. If you die first does she have to give them / me the money immediately? What if this means her selling her house?
+2 = You want a long term relative to have security even if you die. Does she have to pay this for ever more? What happens if she remarries?
+3 = She has people you don't want to leave money to but they are legally entitled to it if you are not there. Can you prevent this?
+4 = You and her and baby are happy. You die. She remarries and her partner persuades her to disinherit Baby P. Is that what you want?
+5 = You have family money that you are happy to share but when you die should she and her (possible) new partner have full access to it?
Points 1-5 are unlikely to happen but so are the chances of winning the Euro millions and Japan getting hit by an earthquake that destroys are large part of the country. A DIY will kit will not cover these points but a good solicitor will.
The next question is, is your mate any good?
If your situation is simple then I would still pay £100 to a certified pro and get them to do it. I would not pay £300 to a chancer with a converted DIY kit.
Google for Wills gone wrong or similar to see what might go bad and ask whoever you do speak to what they would do to prevent it happening to you.
If they say 'It probably wont happen' then leave and ask someone else
*Yes some personal experience