Thanks to all that replied.
We ended back at the Dealers with this problem and the immediate answer was it can't syphon like that because the overflow is spun wealded to the tank, there is no pipe inside.
My responce was "well you try it and see"
So the rig was put in over the pit and the hose connected, the tank filled until it ran out of the overflow and the hose disconnected. 20 minutes later and it is still running out of the overflow and a small flood is beginning in the pit. So the mechanic goes underneath and pronounces that the tank is still full. Rubbish says I, standing on a very wet floor. At that point there was a huge intake of air (like a big fart) and the flowing water stops and there is now only a third of a tank full.
So again the hose was attached and the tank filled up until it started out of the overflow. Here we go again, except this time the mechanic, skinny guy, had disappeared amoungst the tank and all the rest of the stuff around there. He shortly reappears to say that the top of this very wide, but not very deep tank was collapsing as the water went out. So who would have thought that any syphon once begun would be strong enough to do that.
Anyway the elbow where the overflow pipe exits the tank was replaced with a tee piece and a vacumn breaker fitted to the additional leg, which allows air into the top of the tank. It seems to have fixed it, as several fills afterwards were ok.
So there mystery solved.