Hello is this an AST?
Does the contract say you are liable for professional cleaning?
You can always complain to the scheme.
Quite apart from anything else, it doesn't cost £600 to have the property professionally cleaned.
If the property was clean when you left because you employed a person to clean it then it should be a simple matter of producing their report.
The scheme probably won't much like the attempt to make you liable for a professional clean anyway as that is a landlord responsibility generally speaking.
Can I clarify anything for you?