Problems with boiler shutting off after Radiators are used?

Hot water is fine and when we turn on the rads they srtrike up and stay hot, as you'd excpect. However, once the radiators go off the boiler loses all pressure and goes into F25.
It was all working fine before summer but then it started playing up in August after we returned from 2 weeks holiday (and we'd turned everything off while we were away).
Someone came out to look at it and made some minor adjustments. he said there wasn't much wrong with it and something had seized up so he'd sprayed some lubricant on it (sorry I can't be more expansive). He said he couldn't really find a fault with it.

Sinec then when we've gone to put the heating on we've started experiencing these problems.

Someone told us we should drain a bit of water out of the boiler. Does this make sense? And if so how do I do that?