Hello Ian
Just a few comments rather than identification, so probably not much help.
The engine is so typical of many designs both for commercially produced and for home construction, that it is unlikely that it can easily be identified. About the only thing that may help is the cast bedplate, maybe an underside view would help.
I can't believe that the boiler was ever used to steam the engine. A plain pot design with no water gauge was surely intended for spirit firing - if it went dry there would unlikely to be any serious damage. Gas, liquid or solid fuel would NOT be clever. What is it made of? - I assume copper or brass. If it's copper, I would guess that it is home made, but if brass it could be a commercial one, Probably from one of the many plants with a horizontal engine - they quite often used a simple steam cock rather than a proper screw valve. I assume that at one time there had been some sort of casing around it, but there is no obvious means of holding it in place. And like DereK says - is there any evidence that it has ever been fired?
Not a lot of use I'm afraid - just more questions.
Malcolm