As your post was some time ago, I am assuming you may already have an answer to your question about an agreement.
If not though, I may be able to help. I have just had a legal agreement drawn up here in Australia and I used a sort of template to do that. It fits in with all the legal requirements here and i am sure could be adapted for the legalities that apply in the UK.
If you want to know more, let me know – I am username Tori2