Dear Idlestop and Shawn Coyle.
And to everyone also.
The answer to the 'problem' is that there was/is a blockage in the Static system at the point before the static pipework splits to the Altimeter and ASI. Changing to the alternative source had no effect. This provides the error which manifests itself as an incorrect indicated airspeed and lag on thr altimeter due to the incorrect static pressure delivered. Hence the over-reading of airspeed during descent and under-reading during the climb. This isn't too greater a problem when flying VFR but the dangers are there if flying on instruments. However it would be most helpful if PEC correction data were available in cercumstances other than S&L. During my time flying Gazelles IFR and on approaches there was the Aircraft Allowance to put into the computation for IFR arrivals to make allowances for altimeter lag and pitot position errors at that angle of approach. It is always worth checking the relationship of data given by the ASI and that by the GPS. There should be the W/V in there somewhere.
Unless somebody knows differently.