Landlords cannot legally put the rent up just by telling you, or by writing you a letter. Rent can only be increased by formal application, with proper notice, or if there is a clause in your contract about fixed rent increases, or if you sign a new contract at a higher rent.
If you think the rent is too high, you can apply to the Rent Assessment Committee (RAC), who will decide whether the proposed rent is in line with the market rate based on the levels recorded by the Valuation Office Agency (see http://www.voa.gov.uk).
Unfortunately, we have not had rent control in the UK since 1988, so rents are determined by the market, not by quality or by average salaries. You should be aware though that because the RAC only decides whether the rent is in line with the market and market rents are so high, the RAC may give you a higher rent than you already have!