Payment for goods not received

Saint79

I have recently noticed that I have been paying a standing order to a company for cleaning services to a property I lived in in 2009 and moved out of in May 2009. I cancelled the cleaning service in writing before I moved out of the property.

I having noticed I have been paying this standing order since 2009 for a service I have never received I contacted the company requesting payment from May 2009 to date around £900

I received a message back saying that contractually I have to give 2 months written notice to cancel the service they have no record of me cancelling and according to their records are still providing the service.
Not sure how as they returned the key to access the flat to me back in 2009.
I know that it was cancelled in writing and followed up with a call from the owner of the company as we discussed my reasons or cancelling ......bad service
I am 100% certain I provided written notice but as this was 4 years ago I can no longer find a copy of the letter.
They have advised that as a compromise they will provide 50% refund.
Not entirely happy with this as I know I provided the written notice and surprised they think they are cleaning a property they can not access and no cleaner had been attending before I moved out so services had definitely been terminated prior to may 2009.
As this is due to non cancelled standing order bank suggesting they are not able to help.

I haven't provided any response to the suggested compromise and wanted advise on where I stand legally in terms of getting the full amount of 900
Any advise??.