I have been told that I have an airlock in a cold pipe... here are the
symptoms...

Open the tap and it runs a normal pressure (tank fed) for about 30 seconds
then reduces to a dribble then almost nothing. Leave for ten minutes and it
does the same. As mentioned this cold tap is tank fed and it is (to date)
not been possible to trace the pipe. I cannot lift boards as the flooring is
stuck down, the feed comes from 4 huge tanks on the top floor with old lead
pipes reducing somewhere to 22mm copper before it gets to the tap.

Is there a trick I can use?

Tony