Done & Done!
They're in, look great, and work perfectly!
As with Brads install, I had to get a thinner backing plate - but I didn't need to grind anything on the back of the housing. Found this stuff at the local hardware. Not sure what you call it, but it's about 1mm thick, and works a treat.
The other thing is, I had to lengthen the wiring on the LED board to ensure it exited the light housing, making it long enough to get my fingers on it to twist into the standard socket.
I've got some before & after video, so I'll post it up later after I do a bit of editing etc.
Thanks for all the tips people - hope I didn't annoy you all too much!