Thanks to everyone for your responses :-)
I'm glad you too think it's from the sump - the first moment I saw these in the oil I though "WFT, are my bearings going fubar again?" Not a pleasant though
I thought the sump plug was fine - I didn't need any extra torque to take it out, did it just with a regular hex key, and screwing it in seemed ok as well, I did need to push on the hex key in a few places to overcome a bit of resistance, but again didn't seem excessive. Checking the plug visually today after driving to work I didn't notice any oil leaking out, so everything seems good... this time anyway :-D
Pujazuba: Of course I have changed the filter as well :-) When I have a bit of time I'll try and take the old filter apart to see what kind of stuff has accumulated inside. Btw. is there actually a straightforward way to disassemble a filter? Or will I need to just cut it apart with a grinder?