Sorted over the weekend. Ended up going with Helicoil mainly down to cost (65.00 for the kit) and it's what my mechanic recommended as a initial starting point.
The main issue was getting the threaded sump bolt out. Luckily I had changed to the bolt type sump plug (as opposed to the recessed hex original style) a few years ago. We made an extractor tool using a slide hammer and mole grips. An hour later it was out. It would have been much more difficult to remove a hex type sump plug!
The helicoil was inserted smoothly and the cutting tool made light work of the alloy.
Lesson learnt.......do not ever tighten the sump plug to 42NM. That's the value in the Haynes manual
