Smudge,
I know that copies of the 115 Sqn records from RAF Witchford (5 miles E of RAF Mepal) are held by Cambridgeshire County Council (both F540 and F541).
The same may well be true of the Mepal Sqns.
You might also try the 75 Sqn Association:
75 Squadron Association New Zealand | RFC | RAF | RNZAF