Thanks Ian, that's pretty much what I thought was happening. I use microbore and subsequently never quite have enough flow. I also have to use barbed inserts in the pole hose which restricts the diameter of that. On top of all this the pole hose tends to twist and is permanently flattened at the point where it bends around the exit point of the pole base.
As I can't get any more flow by adjusting the clock past about position eight I guess I'm on maximum anyway and the pressure switch is just coming into play now and again. I guess I need to back the dial down a tad.
I have a Flowjet pump and have not played with the pressure switch - probably there is a set screw I can alter.