You're quite right Dave on the terminology. Before my first post on here, I thought I was dealing with a cartridge bearing on the hub. As in couldn't be opened up and repaired. When in reality it's not.
The bike shop opened up the hub, and the problem looks to be that the axle is bent, very slightly. I'm not using it as a serious MTB so the only really heavy impact would have been the car crashing into me 1500 miles / 6 months ago.
The bearings did actually look very clean, and still had a good coating of grease so the seals have been excellent, both against dirt and the slightly excessive cleaning with Morgan Blue when I started looking at the hub!