As suspected, a great big hole had to be opened out in the bottom of the smokebox to enable the boiler unit to sit down properly.
Being whitemetal, my getting it in slightly the wrong place at first didn't matter, a sharp knife was enough to carve the hole in the direction required. It's underneath, so no-one will spot it.
Lashings of C&L 100 degree solder hold everything in place. This includes the pair of triangular supports at the front. The curves are a hopeless match for the boiler and even solder wouldn't bridge them, so there will be filler action later on.
I am disappointed by just how tarnished the brass is looking. It's being cleaned after every session with Shiny Sinks and has even been doused in Cilit Bank. Can anyone suggest anything else? I'm not going down the Brasso route, I'd like a dip of some sort.