Haven't managed to follow all of the description, but 9.5mm
plasterboard, wired to the beam at 100mm intervals and covered with
12.5mm of plaster is deemed to meet the 1hr requirements AIUI. Another
alternative alternative is to use metal lathing (expamet) with 12.5mm of
gypsum plaster over it. Not sure about this, but maybe 12.5mm of
Masterboard? can meet the regulations. try these on your BCO. It used to
be Schedule 8 of the building regs.

Regards
Capitol