try tcd they have started doing odd date flight durations which look very resonable also thomsonfly often have cheap flights. We tend to just keep checking the websites airtours and mytravel are also worth a look.
With scheduled airlines, I find the prices to be cheap(er) from January right through until the end of April - with the exception of the Easter period of course
The other cheap period is from 1st November through to mid December.
Charter airlines may differ (know very little about them) but I've often had good deals on flights that included the February half term.