Had a look through the T&Cs for the Companion Ticket and a quick search around FT and can't seem to find the answer to what is probably a simple question.

I'm going to be booking MAN-LHR-NRT return on a 241 voucher in a few months.

I was wondering if anybody knew whether the classes for the two longhaul sectors could be mixed - i.e. LHR-NRT in F and return NRT-LHR in J.

I should have enough for F for both legs of the trip, but thought NCW would be quite cool for the return and maybe save me a few miles.

Does anyone know if mixed classes are allowed and if so, would there be a saving allowed on the miles redeemed?