I suspect this may be to do with income tax and not the amount of gross pension you are being paid. You will generally have to pay UK tax at source on your pension in either of the following situations:
1) you are not regarded as 'ordinarily non-resident'. If you have permanently moved away to retire you should be able to claim 'ordinarily non-resident' status, so this wouldn't be an issue. However it depends on whether you have cut your links and whether you keep coming back here to visit.
2) the country to which you retired to does NOT have a double-taxation treaty with the UK. You didn't tell us where you moved to.
See here for more indo.
http://www.direct.gov...tepension/DG_10026714
In any event, every March/April the DWP writes to State pension holders to tell them what their new figure for the next year will be. This letter also tells how the taxation at source will work. Are you saying you received no such letter?