Today I managed to get the old stripped rivnut out for the lower strut brace. I ended up making some cuts in the head of it with my dremel, pulled off the bits which took all the head off the rivnut and punched it inwards with a quick tap of a hammer and large screwdriver, only took a few minutes!
The new rivnut went in fine but did take some muscle to get it to compress, it's made of steel! But it's in and tight, the strut brace is back on and bolted up tight - job done!
That's 3 repairs done, 2 helicoiled bolt holes and 1 rivnut replaced.
I also fitted the Powerflex steering rack bush while I had it up on the ramps, what a 'mare of a job but it's done now.
No sign of the rattle/tapping sound anymore, completely gone - weird.