Could it be the Micro Metalsmiths (of Kirbymoorside) kit, I seem to recall they had a cast firebox, boiler, smokebox. They made a really nice high precision reduction gearbox attached to a quality coreless motor long before these type of motors became popular, with drive shaft to a final drive gearbox.
Quite a few of these kits and drivetrain components ended up at what must have been the UK's smallest by square footage model railway retailer ever - Bob Denny's at Sawley Junction, 3 people in the shop and you couldn't move - sadly now long gone.
I bought up all of the motors and drive trains from the proprietor and still have a few sets left, I don't think I will ever get such a bargain of such high quality components again.