If you had been an employee, the answer is the employer cannot do this - it is an unlawful deduction of earnings.
That doesn't apply to you - you have a contract for your services. This is a civil law matter and the terms of the contract apply. Most likely they never told you this, but have you looked at any small print you may have been given or was the contract just verbal?
You will have to argue this using the small claims procedure at the County Court (provided it is under the claim threshold).