Firstly I will declare an interest, because I write wills for a living.
You basically have three options, a Solicitor, a Professional Will Writer or a DIY/ Internet option.
The DIY/ Internet option is generally only for people with very simple circumstances, and whilst cheap it is often just a way of storing up problems for later.
A good solicitor or a good Will Writer should be able to help and advise on something appropriate for your circumstances.
If you go to http://tas-network.co.uk/findwriter.html you will probably find someone local to you.
If you want to e-mail me nick dot ash at tancreds dot co dot uk I will send you some basic info on the things you should be thinking about, costs etc.
If you go for the solicitor option make sure that they are a member of the Society of Trust and Estate Practitioners. This will mean that they specialise in this area.
Regards
Nick