Is there not an engineering shop near you who do this sort of thing regularly, if you can get the vehicle to them they don't charge silly amounts of money for such work, and they'll probably have a serious vacuum for removing any debris that finds it way inside.

An update

Helicoil was hell. The well is so deep it is so difficult to get the tools for the job. I had to install the helicoild 3 times in the end, as there were all sorts of issues. For example I was sold a tap bit for the helicoil with a pilot nose on it, which put threads in the lower chamber which I didn't want to do, but couldn't find the right tap for it. Did this, but then when screwing in the helicoil, only realised after hours of head scratching that it wound all the way down onto the next chamber where the pilot nose had tapped, so it was squeezing the space and not allowing the glow plug in. Getting the helicoil back out at such a depth is a job! Keeping the swarf out of the engine is a job! Breaking the clip off the helicoil and fishing it out is a job!

But very happy to say I have installed the new plug after cutting the helicoil to fit, and started the engine up, no fault showing and it seems to be running with less shaking.

On to the next problems now! Lots of smoke until I disconnect the MAF and the smoke immediately stops! So am confused as to whether this is a bad MAF or a bad turbo! The car goes into overboost/ limp mode after driving for about 10 minutes.