Hi Francis and a warm welcome to the forum. A Cheddar proteus is a serious piece of engineering and it should be taken very seriously!
Although Cheddar have ceased trading the business has been split up to a degree and I know that part of the business, I think the engines, has been taken over by Stuart Turner models and one of the family members has continued the boiler production.
Possibly a good place to ask the question is a forum that deals specifically in model boats and that is here:
http://www.modelboatmayhem.co.uk/Forum_1.htm
If you don't come up with any manuals then ask again and we will have to go through one or two things for you to start off with. It is a big subject though!
Have a look at my steamer build in the Construction section to give you some ideas of the things to be considering.
If you put "Cheddar" into thier search engine you will come up with a lot of usefull links to people in the know and I'm sure one of them could help out with a photo copy.