CAB for a start. Also, if necessary www.lease-advice.org who can give you some advice on the phone. There are regulations the freeholder has to follow in relation to service charge issues & it may be he isn't following them. Are there any other leaseholders in the building? If so can you get them to join you in making a protest & requiring that the regulations are followed?
I don't understand what you say about leaks to the cellar. How could there be so many of them from your flat that it cost you £10K to stop them? I think it's far more likely that the damp in the cellar is seepage through the floor or walls. Nor do I understand what mould treatment can be done from outside the building, unless it is external solid wall insulation - which is a massive & very expensive job in most cases.