Okay - the other day I finally got some aquatic silicone.
I put some on the cracked area, and rubbed it in with my finger, wiped off the excess, then repeat. Once set for a day, I cut as much off on the surface as I can leaving a thin film of silicone.
Just because of that silicone, the cup doesn't sit on properly. So I took as much off as I could hoping that what was left was inside the cracks.
Still doesn't sit on fully as cannot remove the silicone entirely, though - when testing it I find that the cracks in my initial repair aren't leaking, it's just the water seeping in between the connecting parts.
So I give up, it's better than it was it seems - but I will just have to live with water sitting on the skimmer body, at least my skimmer is in the sump.
Got the skimmer installed, checked the pump, etc working... plumbed it all in which is awkward as the space is really limited and I have my pump at the back of the sump (if the skimmer was at the back it would make it awkward later to get the cup off and the sockets behind).
Turned it on - pump didn't start. Grrrr.
Pulled it all out, pump worked. Put pump back into position, checked it: working. Put on first pipe, checked it: working. Put on elbow into skimmer, checked it: working. Put skimmer output and airline on the pump, checked it: Stopped working.
Repeat this pattern a few more times (sometimes I would stop working sooner).
So, I got the pump out and stripped it down to find that the impeller was compromised by something. Something white and rock hard (looks like plastic itself) was on one of the impeller pieces and the mesh is messed up and completely spaced on one side.
For now, I removed the white thing crud, tried to tidy up the mesh and it seems to be working now -
but I am not sure if I should trust this in it's current state?
Currently running now, with the collar on, and the flow tap fully closed, it's already pulling out dirty stuff, can't tell how "non wet" it is yet as I accidentally flooded the cup.
(Silencer is still being noisy too!)