Vaillant eco TEC boiler F22 keeps occurring?

Our ecoTEC boiler keeps giving an F22 error. We top up the system and then it's Ok for a week or so before it reoccurs. There's no sign of any leaks in the central heating system and all the radiators have been recently bled.
When the boilers not in use the pressure gauge reads about 0.6 and when in use it reads about 1.0
However occasionally we've seen it in the red part of the gauge and my suspicion is that when this happens an over pressure valve releases some of the water in the system and thus over a period of time, eventually the pressure drops too low - hence the F22 error.
The outside part of the over pressure outlet pipe isn't accessible (as this is a flat) to see if it's wet.
Does this sound like a reasonable diagnosis and if so, what's the likely cause ?

David Rose
January 2009