Did reply to this previously - but my answer seemsto have disappeared!
In the end, I called British Gas to service the boiler. They found no evidence of a problem. They advised me to monitor the ceiling for any evidence of a stain which might indicate leakage from the flue. Also installed a carbon monoxide detector to put my mind at rest. Still don't know what the dripping noise is, but the engineer suggested maybe it was actually just the sound of pipes expanding and contracting. Am just monitoring now for any evidence of staining or damp....if none appears I will try not to worry about the "drip"!
Jane
June 2009