I have used the small claims court successfully in the past for this very purpose, and it certainly wouldn't hurt to write a letter to that effect.
As for cost, yes it does cost money but I reproduce below from the relevant government website if that is any use to you.
For free legal information, help and advice contact Community Legal Service Direct on 0845 345 4345 or
www.clsdirect.org.uk
How much will it cost and what if I cannot afford it?
The fee you will have to pay to the court will depend on the amount you are claiming, including interest.
You will have to pay a court fee unless:
• you receive Income Support
• you receive State Pension Guarantee Credit
• you receive income-based Jobseeker’s Allowance
• you receive Working Tax Credit with no element of the Child Tax Credit. Court staff will explain this to you.
• your gross annual income does not exceed a specified limit. See booklet EX160A for more details.
Regards.