5 Passengers 4 seat belts

I am going out later today with my OH and my 2(adult) grandchildren and have been asked to give somebody a lift on return, however my car has only 2 rear seat belts and I have said no. Does anybody know legally where you would stand, I have googled this and all I can find is 'passengers must wear seat belts if they are fitted', which doesn't really answer the question - would it be illegal