A little bit further along - I wondered if the noise might be from the bottom bracket rather than the hub, and the noise was being transferred down the bottom tube to make it sound like it was at the back of the bike.
So I removed the bottom bracket and took the bearings out. I found this mess on one side:
I’ve ordered a new set of bearings. It’s important to note that, although the bearings in the bike are labelled ‘6002-2RS’, they aren’t the same size as those with the same name offered for sale on eBay etc. The dimensions are 31(outer dia.)x16(inner dia.)x10(width)mm so I’ve ordered some 163110-2RS bearings instead.