Depending on what yu leaseholder agreement says if you are a leaseholder or tenancy agreement if a tenant ;;
I would think that any overpayment would be automatically deducted from the amount due for 2011/12 just as any underpayment would be added to the amount due.
As I understand it, from precvious contributions to IH, service charges can include 'a sinking fund'/amount for future expenditure (one would assume of a major nature).
You should, I would have thought, receive regular statements of monies paid and the balance owing.
As I am giving you "off the top f my head/inexpert infomation", I may be wrong however I woud advise yu to 'google#' housing advice organisations, many of whom will give free legal advice.