After a rather muddy ride it appears that the cartridge bearing in the jockey wheel has seized. I can’t read the bearing code on the housing, so I’m somewhat stuck. The jockey wheel itself is almost new so it seems a waste to replace the whole thing.

Does anyone happen to know what the bearing size is?

Mine have done this.
Usually pop the seals out.
Spray inside with GT85/Wd40 and work the bearing inner until it rotates freely. Then blow out with airline and fill with a good quality grease(Blue Marine Grease). Replace seals and refit
Make sure you fit them the correct way around

Thanks nicolaisam.
Is there a good guide on removing seals? What tools do you use?

i use a stanley knife blade, or a very small screwdriver

Sewing needle works well, as does a scalpel blade.