According to the Evening Standard:
"This tension will be at the heart of the debate when British MPs vote on an increase in the IMF contribution. A big Tory rebellion is inevitable."
Cameron is quoted by the BBC:
"(the last vote had) allowed for some extra headroom and what we would anticipate doing would be within that headroom."
Ergo, if the last vote contained sufficient 'headroom' to allow Cameron to increase funding to the IMF, how can there be any further debate or vote in Parliament? Even Douglas Carswell appears to be slightly confused here, where he intimates that a vote will be forthcoming on any increase in the UK's contribution to the IMF.