I've answered my own question, but for future reference the answer was 4xM6 holes and 4xM8 holes per set of 2 mounts.
I've attached the relevant instruction for the Bedford MJ:
TRUCK, 4 TONNE, 4 X 4, BEDFORD MJ (ALL VARIANTS)
MODIFICATION INSTRUCTION No 20
Sponsor: Publications Authority:
DGES(A) Vehs & Wpns Br REME
Project No: 71712(169)
File ref: 0736/71
SUBJECT: SA80 rifle clip mountings
(12-1819)
Hopefully it will be of interest to some of you!
Stone
SA80_Mods.pdf