non driveside arm working loose, little bit of wear on the splines, short of buying a new crank arm, would a liberal dose of threadlock help keep it tight?

What sort of crank is it?

Middleburn

What cranks, HT2 you could bin one of the spacers to free up some extra spline, replace the plastic pre-load cap with an ally one (ebay 4 quid) and my plan for something similar is shitloads of Loctite 641 for bearings as it’s supposed to be able to be undone again for servicing if needs be.

Just seen your post I had to bin my Middleburns when they did this but still worth a go I suppose.