It seems I’ve wasted my money by buying a pack of commonwealth bogies to update my mark 1 coaches; these are the current ‘blue riband’ type.
It appears the bogies are designed for the original type of mark 1s produced by Farish, with bogie-mounted couplings, as they have the clip-on type of coupling mount, no ‘extension slot’ on the pivot hole and no taper to the outer end to allow a NEM pocket to swing. Rather disappointed to say the least.