"Are all of the 80 hours aeroplanes credited against a CPL/IR with a frozen ATPL? Or only a proportion of those hours counted?"
I might be wrong but as far as I understand it from my team, there is no "conversion" between MPL and ATPL... so there is no credit for those flight hours
Would love to hear I am wrong and explaining how the credit can be done
The answer is at FCL.325.A
Before exercising the privileges of a CPL(A), the holder of an MPL shall have completed in aeroplanes:
(a) 70 hours of flight time:
(1) as PIC; or
(2) made up of at least 10 hours as PIC and the additional flight time as PIC under supervision (PICUS).
Of these 70 hours, 20 shall be of VFR cross-country flight time as PIC, or cross-country flight time made up of at least 10 hours as PIC and 10 hours as PICUS. This shall include a VFR cross-country flight of at least 540 km (300 NM) in the course of which full-stop landings at two different aerodromes shall be flown as PIC;
Note that these requirements apply to the
holder of a MPL, there is no mechanism for crediting hours received on an incomplete MPL course towards the requirements of a CPL. Note also that PICUS, within the meaning of FCL.010, may be gained only when acting as co-pilot of a multi-pilot aeroplane; time gained as SPIC or (peculiarly in the UK) P1/us may not be counted.