Therefore if I was replacing the floor (and we'd thought about slate) then
I'd want to take the lot up and stick some particularly meaty, flat
external
WBP ply down, fix down firmly and then lay on top of that.

Sounds like too much work to dig up the old stuff, although the ply on top
might be good. The kitchen units are already going in. I can't live without
a kitchen any longer and only have a couple of weekends free to fit it. The
floor will probably be done in 3 or 4 month's time. I can allow height now
when adjusting the units, appliances, and worksurface height if I know how
thick the floor will be. I won't be tiling under the units.

So, in terms of height, we have:

4mm hardboard (can't be bothered to remove).
6mm WBP plywood
3mm heat resistant compound
3mm Underfloor heating
4mm Flexible floor tile adhesive
10mm Slate

Giving a total of 30mm. That's going to be one annoying step, unfortunately.

I've no idea what heat resistant compound is, but I'm told I need it. No
idea how thick it is really.

Christian.