2 ball valves from b&Q look identical ones high pressure and ones low press?

The only difference is the plastic seat inside the ball valve (the hole bore going through it are either larger or smaller) As you point out, use the high pressure for mains water and low pressure for gravity. I think what your are neglecting to do is to adjust the ball valve (arm) for the water level to finish below the overflow outlet. If it is brass then physically bend it down with your knee half an inch or whatever it needs. If it is plastic then use the adjustment screw near the body of the valve.
Good Luck.

Pete the Plumber.
December 2010