hi phil,
i test all my cylinder assemblies on air when setting the valves. my machining and skills are very average and my equipment very basic and antiquated. when on test under air, no sealing is used, just metal to metal. make sure you have removed all burrs etc.
on final assembly i add a smear of old fashioned red hermatite, or these days one of the expensive car part copper silicon sealants. i never use gaskets on the cylinder assembly except in one special application that may be relevant to your Boxhill which i can supply you with details if required.
for boiler stuff again metal to metal initially and i then add a 1/64" gasket of the modern type now sold by Blackgates. i mark everything up with engineers blue before the above. the addition of a gasket is a bit of 'belt and braces'. a large inner dome flange will frequently slightly distort when silver soldered to the barrel and needs blueing up and truing/fettling afterwards.
a very experienced miniature loco friend of mine plus boiler inspector uses ordinary silicon sealant on boiler parts bolted together.
hope the above is of some help.
cheers,
julian