Thanks - I need to look harder before posting in future… I can now see some proper ‘Helicoil’ items on that well known auction site hiding in between the thousands of other brands…
Actually just bought some ultrasonic cleaner from Zoro…
At the risk of incurring groans of “hasn’t he STFU yet?”, here is some more useful info
Every day’s a learning day 
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And on the sidebar are links to the other thread repair methods that have been referenced so far.
OK I’ll shut up now…
Just my 2 penn’orth, re my final drive box, I went to an engineering shop and they decided what they were going to do, with good reasons, which they explained. Go show it to an expert, saves a lot of guessing and 'what if’s. 
A sentiment with which one must agree.
When I spent my early years as an engineer doing helicoils, the brand helicoil was owned by Armstrong patents. They where very expensive - a 1" tap was £100 in the early eighties. There was a roughing tap and a finish tap for each size. Nowadays a ‘helicoil’ set is cheap as chips and only has a finish tap, but gets the job done.
Timeserts came out in the early eighties doing a similar job as a helicoil, but where a solid insert as opposed to a wire wound spring. This meant a needing a thicker tap. So to prolong the life of a threaded hole, do a helicoil repair first, then if that fails use a timesert, not the other way round.
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Or drill and tap a larger size and use a stepped stud instead.
If short on space at the rear to add a nut, use a countersink bolt and countersink the hole. You could keep the thread diameter the same then.
Thanks everyone for all the useful information and suggestions.
As luck would have it, a friend I’ve not seen for a while, but caught up with recently, and who nowadays runs engineering training courses has said that he will be able to get it sorted out in their engineering dept, but they will need to take a look at it first and decide what to do. That won’t be for a weeks yet though, but I’ll report back on the outcome just for completeness on this thread.
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Final Update: Timeserts were used and I chose to get both sides done (all 4 threads), and so far they are excellent!
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Glad to hear it