Just went and took a look now (needed a break form marking exams - hooray for holidays, hey?)
Obviously physically v similar, same dimensions and so on. Existing one wasn't original, or had at least been off at some stage - lots of messy gasket goo along with a metal gasket. New (aftermarket) pump came with a cardboard gasket. Blades different shapes (pictures to upload later).
The hydraulic clutch in the new aftermarket pump was significantly stiffer than the old one, whether that's age or design differences. I assume that's a good thing because it will kick in earlier than it might otherwise? Or at a cooler temperature which?
Anyway, best news is that the galleries in behind it were very clean.