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Is there no slack in the wall? If not you could (carefully) cut a the insulation out of the way and connect with straight wagos. It should be contained in a maintenance free box according to the letter of the regs but the fact that it remains accessible if the back box is removed means you could bend that rule a little bit. Proper way to do it would be pull a new piece of wire through. Sometimes that's easier than you think it would be.
what's this fix for this little fuck up, other than opening up the wall (wood cladding and OSB on the outside, plaster board & PIR on the inside)?
I drilled a hole, and completely neglected to consider the live wires just on the other side.
The short can from the drill bit itself - there's no (apparent) short in the wire now.
Obvs I'd rather not open the wall up, which would be a giant pain, but I'd also rather not burn down the entire building by bodging an unsafe fix.