One of the cars has fairly new discs but one of the threads in the disc (a Peugeot) for the wheel bolts seems like the thread has been stripped and the bolt only goes hand tight before turning in the hole (have tried another bolt).

The wheel bolt is M12x1.25.

Save having to spend over £100 replacing two perfectly good discs and pads, is it an option to repair the thread with a helicoil/thread insert or to re-tap the hole as M12x1.75 and just get a different wheel bolt for a couple of £ until these discs and pads need replacing? 2015 Range Rover Autobiography 4.4 SDV8

As long as it is done properly a helicoil should be ok. We use helicoils for some critical stuff at sea, never had a problem with them.

Excellent. I'll whip off the disc and get it on the pillar drill. 2015 Range Rover Autobiography 4.4 SDV8

If it helps, I believe the wheel bolts on Ifor Williams trailers are M14. Might help if your stuck.