I agree with Dave, Stuarts for a while were dealing with cheddar plants (which I don't think they are now) which included both boilers and engines (until I bought their engine stock) They are well made boilers, and looking at the pictures that looks like a very well made D10....if thinking of buying...do not go to high a price.
I do valuations for some auction houses but cannot put a value on the plant without seeing it but I can give you a guide figure if you need it (please PM me)
Picture of one that came through my workshop, really well made. Hope this helps