Thanks to everyone who has responded, but I think I will go with a slightly bigger boiler. I’m awaiting the drawings for the Kennions Vertical Boiler, I also have the book by Alex Weiss “Making Small Gas-fired Boilers for Steam Models”. I have built the hand pump as detailed in parts 5 & 6 of the original series, but instead of using steel balls for the check valves I used poppets with O rings. I have just used it to test a little pot boiler to 80 psi, for another tiny engine I’m building so I think it will be OK for any bigger boiler that I build.
Ken