Tim S coughed up some electrons that declared:

Ball have an epoxy DPM called F76 - have you come across this? Claims to
deal with rising damp (if any) and it's not *that* expensive. There's also
Stopgap 900 which is used under F76 should the substrate be too bumpy to
coat well (that'll be the old wall base and the bit in front of the fire).

Then P131 and Stopgap 300 to top off.

Wonder why they didn't mention those yesterday - I'll ring them again and
put it too them. Need scarifying but with the ease the old crap is coming
off, taking much of the Blackjack with it, that shouldn't take long with a
suitable machine.

Ideally, I'm keen to use a system that finishes with a product I know, for
obvious reasons

OK - good news...

After a chat with F Ball, the system that they said would work is as
follows:

Scabble/scarify to take of all of the black junk;

Smooth off any very rough bits with Stopgap 900 direct to base.

2 coats of F75 (variation here on my initial theory)

P131 and Stopgap 300 as required to flatten.

They said that will cope with any damp that may be present, and stop it dead
and will be happy under an impervious covering.

Now I need to go finish cleaning the floor, take a survey of heights and
estimate materials, but initial measurements suggest 15kg F75, bag or two
of 900 and perhaps 6-8 bags of 300 and 5l P131.

All in it's shouldn't work out too bad on cost...

Cheers

Tim