You accepted the salary, so there is nothing you can do now.
This following information applies to the US. I don't know if it applies to the UK where you are, but it might apply to you. You indicated that you were in a contract position. If you are contracting through the recruiting agency, be aware that some companies will negotiate a rate with the company, then negotiate a rate with you. The recruiting company gets to keep all the money between the two figures. So for example, if they agreed to charge the company $100k for the year and agreed to pay you $80k then they would make a gross profit of $20k, which is their normal profit. If they get you to agree to a salary of $75k then they get to keep the extra $5k, making their gross profit $25k instead of $20k. This would only apply if you are contracted through the recruiting company, or a company under their control, but would not apply if you are contracted directly to the company.
Hypothetically, let's say the recruiting company's normal cost of placing you is $15k (salaries, advertising, commissions, etc). That would mean that they normally have a clear profit of $5k. If they can get you to take $5k less, they have now doubled their profit to $10k, so these recruiting companies have a strong incentive to do this. This happened to a friend of mine.