There should always be a letter before action.
Do you have a copy of the service charge demand? If it doesn't include a list of your rights and their obligations under CALRA it's not valid, and you can effectively consider yourself not to have been served correctly - ie you don't have to pay until they send it properly.
CAB suck at this stuff but https://www.lease-advice.org/ will analyse your lease and tell you what is and isn't payable by it. Usually admin / legal charges like this aren't payable without an explicit clause - and there usually isn't one. Book a telephone appointment if you can