I think ive worked out how to build a very cheap anti syphon pipe.
I need 3 pieces of pipe and a T piece, put pump in water and T it off just after the pipe leaves the tank, then have 2 pieces of pipe running to the sump, if only 1 of them runs under the water in the sump and the other sits higher than the water then if the pump stops the pipe will empty itself as air can flow into the higher side.