"I have always understood that with VNAV engaged the manoeuvre capability is 1.6g to max.lift coefficient at .80M for CAA aeroplanes and 1.3 for FAA aeroplanes.But when selecting the MAINT page of the FMC on our CAA aeroplanes the manoeuvre margin reads 1.3."
Are these values (1.6 and 1.3) 757-specific? On the 747-400, for example, it's 1.2 for FAA regs and 1.3 for CAA (British and Australian) regs.
Note that for some aircraft, updating the monthly Nav Database can result in the Maintenance Page figures resetting to default values(e.g. 1.2 for Manoeuvre Margin for the 747-400). It's up to Maintenance to ensure that the correct value is in there for your particular airline after software updates have been carried out. This value should not be adjusted by the pilot under normal circumstances.
BTW, doesn't your airline have a software sheet say, in the Tech Log, which tells you what your values should be? If it is supposed to be 1.6 on your aircraft, I'd be asking Maintenance about it.
Rgds.
Q.