Tropicopter - many thanks for the explanation - I feel heaps better now, I think.
No, I'm going to stop thinking, especially about your explanation; maybe I am a total prat after all; Boyle's law's easy, so is (are) hydraulics; theory of flight's a doddle; but I'm not sure if you're winding me up or if it's an arty thing like "look at the pathos in this pile of bricks..." but I'll give you the benefit of the doubt for now, so Thanks again.....
Clearblue - I wish I'd said that 'cos it's exactly wot I Thort (je suis relatively illiterate engineer type)
Ho Hum.....