Hi Paul,
Here is the explanation from a Camden Collections Officer.
In the absence of section 20b notices which should be issued every 18 months since costs are incurred, the landlord is limited to recovering an amount capped at £250. The 18 month rule could apply if your contribution was a demand for over £250 however in this case you have been charged under the cap at £212.22. Your contribution is therefore recoverable under section 20 practices.
Mick