my “new to me” 2003/53 Disco I’ve found that one of the rear bolts to the rocker cover won’t tighten. Presumably a chimp has f****d it. Is it the end of the world to just leave it ? All the others are fine and tightened up to the correct torque

If it's not leaking then I'd say it's fine to leave it. You may try to take that bolt out and see if it's the bolt that is stripped and not the bolt hole... I've seen quite a few Td5's with the wrong bolts put in there b/c they get lost when mechanics are working on them. The torque on those is only 9 NM or 7 lb/ft, but in the attempt to stop the rocker cover leaking they get incorrectly overtightened, which IMHO just makes the leaking worse b/c it distorts the gasket. Maybe swap bolts with another and carefully screw one in and see if the same thing happens. But if all are torqued correctly I'd say one loose isn't too big of a deal.

Cheers, yeah it tightens a bit but then just spins but doesn't get anywhere near 9nm. I'll try swapping them around if the bolt looks ok, but didn't want to leave it if it's going to leak.

For sure the aluminium thread has stripped rather than the steel bolt.

If it's not leaking, all well and good. But I think you'll need to Helicoil it when the cover is off next time.