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And the ultimate winner is.... Malagabob with Buildersmate close behind. The solution was a bit of both.
Sadly inserting a boss into the existing didn't work, due to the sneaky presence of the lower curvature of the toilet branch on the right. Meh. Five quid I'll never see again.
So.....out it came, root and branch (hur hur), replaced with new access, new pipework and a lorra lorra bad words. The weather alternately pickled down or was too warm, y'know the scenario, sweating like a pig inside the waterproofs. But it is done. The iso valves have been turned back on, nothing leaks. I feel bliss.