Thanks for all your responses, guys, much appreciated!
It's definitely not the original pump, as the silicone sealing looks quite fresh. I guess the previous owner has just fitted one of the crap ones you're talking about.
However in regards to water pumps many Porsche owners seem to agree with what brian325is is saying - that some aftermarket Pumps with Uprated Metal Propeller (whatever this means

) are better than the genuine ones. And with prices varying between 70 and 300 pounds, it's not an easy choice. I'll probably go for an OEM though.
I'll ask the mechanic to have a look at the manifolds and pipes as well.