Hello, my name is ***** ***** I am a qualified lawyer happy to help you today.
They can try to ignore you but they would be foolish to ignore court correspondence. As such, I would recommend that you send the agency a letter before action to demand payment within 14 days and say that if they do not pay you, you will issue county court proceedings against them. See attached template as an example of what to say. You may also want to threaten a report to the Housing Ombudsman for good measure.
You would then need to register at http://www.moneyclaim.gov.uk so that you are ready to issue the claim in the event they dispute the claim and do not pay you. The website is very user-friendly and you would not need a lawyer to use the money claim site. Claims with a value of under £10,000 are classed as a "small claim", so legal costs are not recoverable and the matter would be dealt with on paper by a Judge, not at a hearing. Claims between £10,000 and £25,000 are subject to fixed costs only so if you lose then the risk is minimal. Further, the money claim website allows you to sue for an amount up to £100,000.
You would claim the sum for the loss, the court issue fee (details of fees are here at page 5: http://www.gov.uk/make-court-claim-for-money/court-fees) and court interest which is 8% calculated on a daily rate from the date of loss to date of court judgment.
Once you have the CCJ from the court the agency/defendant has 14 days to pay in full. If they do not then it gets registered with the credit agencies after 30 days. You can also enforce the CCJ with the county court bailiffs or transfer the debt to the High Court for a small additional fee assuming the total amount owed is at least £600 and you can use the high court enforcement officers who have greater powers than county court bailiffs. The transfer fee is added on to the debt and payable by the defendant.
There are other enforcement methods which I can help with, including bank account freeze, charging order on their property, summons to court for questioning, etc.
I hope this helps - if you can please accept the answer and give me a 5 star rating (there should be a button on your screen to do this), I can answer follow up Q&A's at no extra charge and Just Answer will credit me for helping you today.