I had a similar issue on a paid stay at CP Kensington, booked 2 adults and child using adv purchase rates, checked my upcoming reservations a week later and it had been modified to 3 adults and a child with an extra GBP40 per night added for the additional adult. Called PC, they could not fix "system would not let them", called hotel - front desk was too lazy to bother and said to call PC, called Ambassador services who easily fixed it. Booked twin club room which has 4 person capacity.