Here's the situation. I have some ring main cables coming out of a
concrete
cladded wall.

[ ... ]

I bared the wire in parts while doing this, so have had to replace that
patch of cable.

I now need some safe and reliable method of joining the old ring main
cable
to new.

The best answer is to replace the whole of the run of cable which is
najjered with new stuff. It's a ring main, and Therefore each individual
run of cable's usually pretty short - from one socket to the next.
Really, truly, the 'do it once, do it right, then forget about it for
30+ years' approach is to work out where the cable you najjered runs
from, and replace the run with a new bit.

I entirely agree and I would run new cable if I could. But you missed one
point above - the wires are coming out of a concrete wall. I've no idea
where they've come from. They might have come from the room next door or
they might have come from upstairs. I haven't got a wiring diagram for this
house. I don't even know if it's a radial or ring man system (the cables
throughout appear to be 2.5mm so I'd guess the former).

This really is a situation where joining cables is the only sensible
solution, short of rewriting the entire house.