i couldn't help but notice that some leylands have holes in the storage boxes and some don't, i'm assuming that's for ease of access to mount retractable seat belts ?

how have you installed retractable seat belts into a clubman ?

I once tried to fit retractables into a `67 Mini that had Clubby headrest type seats. The seats fouled the reels, so I fitted std belts.
This car when we got it had been fitted with retractables just bolted to the bins, not captive nuts in the inner sills below the ashtrays. I considered this a safety hazard- in a decent accident it would probably tear the bins off the floor where spot welded.

I was by a friend that the pre 1975 vehicles had retractable a mounted as you say in the bins standard, do yours not look standard ?

No, Clubbies (and roundy Minis) pre 1975? had fixed front belts, no retractables. Rear bin top details are different, no captive nuts in there. Only the front ones, below the doors.
I now have retractables in my 1970 car, mounted in the original anchor points below the doors. They work fine.
The problem with the `67 car `Barney' I mentioned was the PO had fitted Clubby front seats, and their tilt mechanism fouled on the reels. Couldn't push the seats back.