Back dating of a rent increase


Can anyone offer me any advice please or point me in the right direction for a website etc.

Our landlord has just emailed me over the weekend to say that they have increased the rent and that it was effective as of the 1st November, so I must pay them the difference for the rent paid (monthly by direct debit) for the last 5 months.

No problem with the small increase (also where can I check that it is in line with the correct band increase etc.) but can they really make us pay for the last 5 months because they forgot?

Many thanks for any help