Sorry, spoke to James Geddes this morning and he hasn’t got any Slough headlight mounting plinths. Apparently it’s quite common for the bolts to shear off them.
If your careful you can repair them, however there are 2 types, earlier ones made of brass (stronger) and later ones from a form of white metal alloy which is more fragile.
If you careful you can drill and tap out the broken stud and replace it, originals are Imperial size whitworth thread but you could tap out to 8mm and use a set screw and once tight (maybe use a bit of ‘locktite’ as well), cut the head off to length.
The white metal alloy ones need a bit of care though as the corners can break away when drilling/tapping.