a housemate in an identical-sized room has been paying £140 per month less than you for the past 2 years. Intensified by the fact that they moved their boyfriend in without so much as a mention a year ago so essentially should have been paying MORE rather than less rent.
1. Storm straight up to them and ask them 'what up homes?!' and make a totally futile demand to write a check for the near £3,000 over the odds shelled out thus far.
2. Realise that you accept the terms of the rent as they are presented and so, despite being ethically whiffy and showing a certain weasally side to their character, technically it's fair do's and take it on the chin.