Matthew's Journal

As mentioned above, you have very little power to negotiate reasonable terms between you and your letting agency/landlord. If you quibble they can always refuse to let to you, leaving you homeless.

The law recognises that individuals are often at a disadvantage when negotiating contracts, and that as a result contract often come to contain conditions which are unreasonable. If such a contract is challenged in court, any clauses deemed to be "Unfair" may be struck out and held unenforceable.

http://www.oft.gov.uk/Consumer/Unfair+terms+in+contracts/what+are+unfair+terms.htm
http://www.oft.gov.uk/NR/rdonlyres/36623FC6-E9BC-4D9A-B4BF-39191E06F802/0/oft381.pdf
http://www.oft.gov.uk/NR/rdonlyres/DAAEFE58-1AAB-422A-AFED-BDE6C654A4EE/0/oft356.pdf (this one looks useful, especially around page 27)

Of course, it keeps happening because few people choose to challenge them.