Pressure drop problems with Vaillant thermocompact VC GB 282E?

Dramatic pressure drop began with last cold snap about two weeks ago - water churning noise overhead (boiler in former attic space, now part of the flat) woke me at 7 when boiler comes on - pressure at zilch. Let pressure in up to near top of green bit and ok until next morning (i.e. ok for next afternoon/evening session). Same problem next morning and for next few mornings. No sign of leak, no air in radiators. Problem went away after 5 or 6 days, just in time for British Gas postponed visit! He did no testing at all and just said call us again if it happens again. It happened again this morning - water churning , pressure at zilch - no leak anywhere. Any ideas?