[Derek Emblin]

Its the command-trac system.

I'm having some difficulty locating where to reply to all the really helpful suggestions I've received, so I'm doing it here!
Guys thank you all very much for your input, it helped me tremendously, I now understand fully how the system operates, something that the workshop manual I purchased failed to do. What had be baffled was that I managed to get it to operate when not under load, so with the aid of the diagrams and hints I received I deduced that it was not getting sufficient vacuum to slide the collar, so starting from where I had stopped cleaning the pipes I began systematically to remove them and ensure they were clear, and lo and behold I found a blockage in one of the metal pipes, which made it extremely difficult to clear as they are bent every which way. Once cleaned and reassembled it worked like a charm and I was able to go and play in the deepest sand I could find without a hint of bogging down. Again, many thanks to everyone who came to my assistance, I really do appreciate your help. Derek