Well, I had to disassemble the rear of it where the insulation was located.

The previous owners had run it "dry", i.e. run fires in it without the boiler having any water in it, so this beautiful heavy-gauge copper boiler had a great big hole burnt in it, so that had to go. The plan was to replace the boiler with firebricks, so,

1. the back panel had to come off. I swear it hadn't been unscrewed since it left the factory in the 1950s
2. the insulation had to come out (this was the scary bit). It was still a bit nasty because it was all kind of decomposed. I think it was originally in sheets, but it came out in handfuls.
3. the boiler had to come out
4. I had to measure up the void and cut firebricks to fit (this is also scary, because firebricks need a diamond saw to cut, and you've *got* to keep it all wet because of a. heat, and b. danger of silica dust
5. fit the bricks and secure them with refractory cement - also a silica dust risk when mixing
6. apply sheets of rockwool or fibreglass insulation
7. screw the back panel on
8. test fire.
9. damn, it leaks, there is smoke coming out everywhere.

I eventually got it working, though. It's not nearly as efficient as the one in my kitchen, but then there's about 40 years of design and manufacturing improvements between them.