Thanks for all the ideas!

It is indeed from the LBS, they're replacing the cartridges and going to totally pack out them out with grease! I'll keep a bit more of an eye on them in future too.

As for washing, I wash the bike in the same way as my road bike which has done a year and a half and one winter on the same set of bearings. So I don't think it's a case of de-greaser getting in, unless the seals aren't as good as those on the road bike. The only thing it could be is that over this winter I started some WD-40 after cleaning to get rid of water and stop flash rust while it dries in the chilly old English winter.