Ahh! No matter what I do, I can't get the system to work safely. The sump and weir/drain systems are just not designed to work together. The return does syphon water down, past the top of the weir, which was overflowing the sump if the return was turned off. I removed water from the last chamber of the sump to the point it left the return pump barely covered. That gave me the capacity to turn off the return, and it just fitted all the water in, but only if the skimmer was still on (so keeping some water up in the foam). If the skimmer goes off too (in a powercut), then there's not enough space. I then made a stupid error, and drilled a hole in the return, just below the waterline (aiming to stop the syphon sooner) - only thing was, I did it where I could reach, in the top of the elbow that goes into the water. That just shot a jet of water straight up and out the tank (I covered it in case), so that's ruined, and I need to replace the end of the return. I thought I had spare bits in case I needed them, but turns out I have 3/4" and the return is 25mm, so I'm stuffed until I can get hold of some fittings.
Does anyone know if 25mm speedfit will connect to 25mm rigid PVC? I can get that from B&Q in the morning, but would have to wait until Monday to get the propper PVC stuff for a long term fix.