Looking at the surfaces of the cracks I can only conclude that the quality of the casting isn't that great. It looks rather brittle with quite a lot of crystals in it. And yes it was an aftermarket item, not an original Unipart one.
So while I was working on Renés Dolomite today I rummaged through his parts attic and found a pump with a fairly good impeller and a completely knackered shaft (missing a few teeth). Ideal, as the shaft of the pump I wanted to recondition is in pretty good shape. And this impeller came off without a problem.
As you can see it looks a bit rusty but that’s only very light surface rust. Only thing that needs to be done is machining the mating surface for the seal lightly to get it smooth again.