Just an update, may help others with the same situation....
Took the 3D ski to Alan at JetShed, Sheffield, who soon diagnosed the problem as a faulty rectifier, so as I had guessed there was nothing wrong with the pistons. Alan tried the ski on his lake with the original rectifier which produced the same symptoms as I had experienced, fitted the new rectifier and sure enough problem cured!
Can recommend Alan to anyone who has an annoying fault, on any make of ski, and will be using him again rather than the official SeaDo dealer who couldn't help me, and wasted so much time over the summer.
Thanks again to everyone's help and for pointing me in the right direction of JetShed.
Just need some weather now to use it again!