Hi Paul, I think I know what you're referring to and maybe a way round it. When I did my HowTo on repairing the numberplate lights, I mentioned that the plastic pin fixings on the outer ends of the plinth snapped off. The plastic brackets for the steel bolts also snapped, so I took a good look at it all. The inner pair of fasteners are, I think, what you require. Here's some photos:
Here's one of the brackets I fixed using epoxy resin.
- and here's that special bolt
If you're into DIY and have the appropriate tools, a functional head could be produced quite easily by cutting/grinding down an M4 hex headed bolt. Alternatively, a surf through eBay found these:
https://www.ebay.co.uk/itm/M4-Ultra-...UAAOSwLNhgFdMi
Maybe a bit thin, but the head is similar and might work OK
Plinth/Numberplate Light HowTo (see post 4 here):
https://www.the75andztclub.co.uk/for...ad.php?t=78539
Hope this helps.
TC