Fantastic, I hadn't realised even the sealed bearings were a replaceable cartridge. Although looking for my hub on the Shimano website I'm sure it's an FH-M8000 which isn't sealed bearings anyway.
I'm short of a few tools to do the job (chain whip and cassette removal tool) and have seen one of the local shops has a £30 price posted online to replace wheel bearings, so quite tempted to go for that.
But hopefully either way that's a lot less expensive than I feared! Thanks all, and thanks for properly posting my link for me!