tosh mate you are putting a load of pressure on one pump mate. why not have two pumps and one controller and a splinter after them.
It's all sorted now. Two pumps, two pump controllers. The problem was that I had positioned the pumps too high which was creating air locks, and - due to my inexperience - I didn't know if it was the pumps, the airlocks, my hose, or the pump controllers messing me about.
It also worked perfectly as a one-man set up with a high pump, but when I added a second pump, it messed me about.
Anyway it's all sorted now; I just lowered the pumps as low as I can (to the floor) and it works brilliantly. I'm very pleased with it. I'm also pleased it buggered me about, since I tried various set ups and I know which I prefer (pumps with flow controllers).
The real test will be at work tomorrow, but I'm confident it'll all be okay.