I had forgotten that I had already used an RSU to silver solder bits onto the Garratt.......
(From another thread) ........It will (under some circumstances) - on the Garratt, there is a step mounted on the front of the buffer beam, with the lower step mounted off the back of the buffer beam directly behind it...... I didn't spot this'll until I was fitting the last details. Of course the solution was to silver solder one of them on and soft solder the other one on - but I couldn't risk using a torch, as there were white metal castings immediately behind, and all-sorts soft-soldered together.....
So I got my old RSU out and silver-soldered one of the steps on using that (in an effort to keep the heat as localised as possible). It worked, at any rate.