Thanks for the responses.
It sounds as though I'd be better off doing the FRA-LHR-MCT connection (BA 901 to BA 073) in the morning and avoid potential complications of an overnight stay. Does anyone know how long before departure do you have to check in in FRA? I'll have one bag to check through to MCT and I'd prefer not to use the OLCI/bag drop option but actually see it tagged 'short' by the check in agent. I assume that there will be a separate J check in at FRA.
Thanks again.