Dave, people here have given you good advice, to the best of their knowlege.
You state:
"When we go out to look at the next boat I will cheerfully pay petrol money, beer and dinner to any member in good standing here willing to come along and give us his or her opinion on the value of the boat. That's how desperate we are".
I know a little of boats, when we were looking for one we spent some 18 months searching, this enabled us to identify "The Good, the Bad and the Ugly" and get a fair idea of values of different craft.
Going by the tone of your entry above, I for one, would be unwilling to accompany you in case I incurred your wrath and was added to your list of "vermin".
I feel what you want doesn't appear to exist in the canal environment. As I said in a previous post Google "Marine Laywers" and take your pick.
Best wishes with your boat purchase.