The landlord (and his solicitor) would seem to be claiming that the interpretation of 'costs of upkeep of the block' extends to more than just the direct costs incurred (i.e. it also includes the overheads associated with block maintenance). That seems like it is worth challenging.
Is this a 'one-off' associated with the one-off purchase of the freehold? - or have you gotten into an open-ended arrangement in perpetuity where you are liable for payments (even though you own the freehold)?
I wonder what these payments cover, and what formula you agreed (or didn't) as to their allocation. Also, if this is open-ended, just who advised you that it was a good idea to do this.