I managed to borrow another o-ring (square) and with two in place the push fit seems a lot tighter. A liberal application of PU40 and I'm convinced the input side of the pump is airtight. Still no water coming through.
In desperation (lift in was this morning) I took the cover off again, cleaned up and sanded the outside of the it, and put it on backwards. Didn't get a chance to test again before going in, so delightedly surprised to get a good flow within a minute of starting it up.
So looks like it was a combination of air leaks
in both the push fitting and the pump cover.
Incredibly chuffed to have it working and thanks again for all the help. Will look at moving the siphon loop later and probably also get a speedseal.