So, I got the car back last Saturday.
Got a garage to do it iN the end, as the price was initially suggested as £150-ish, ended up been £100 more.......was not impressed, but apparently the inlet manifold had to come off and some modification of tools was required.......to be fair I can see this being true, it is a deep plug and an awkward spot.
Oddly any garage I spoke to I asked about a helicoil and they all replied that they wouldn't use that, that they'd use a different insert, likes I guess either experience, or pub science has lead them to lean towards a solid insert.
Options suggested where either snap on version of time sert, or time sert itself. In the end a time sert went in.
Car is all good now, and defo up on power........it goes to show how a gradually declining part can affect performance drastically without being too noticeable, if it's a slow decline.