thanks dmmsta,
Seen the rubber grommet bit, which is a weather protector by the looks of it.
Looked at the rest of it and all seems very obvious, except one small piece. In the car there is the wire controller that normally plugs on to the bulb pins, and i see a design flaw of sorts.
Once the grommet is in place, the red & black wire are inside that and need to come back out to be connected to the original controller.
Also a 25mm enlargement isn't strictly necessary as this will fit in the outer metal ring.
Finding difficult to explain so will have a go tomorrow and upload some pics of what i mean as the light will be better.
As a side,
How did people manage to get a drill and bit in the non-existent space?
Thanks for patience.