What do I do now? It seems a little strange to have the RCD and
non-RCD MCB's fedfrom the same 'Live' Busbar. Do I have to cut it to
length?

Yes. Decide where you want it split and cut the bus bar appropriately. If
you have any external circuits, such as sheds, garages, outside sockets etc.
it is recommended to fit them on the non-RCD side and use an RCBO instead of
an MCB. Also, you should have a separate 16A radial circuit on the non-RCD
side just for your fridge/freezer. Your lights and smoke alarms MUST be on
the non-RCD side.

These steps will reduce the both the likelihood and consequences of nuisance
RCD trips.

Christian.