Thanks - worth a try.
I tried calling the people I got the replacement carts I have from, , who just said I'd had them too long for a claim.
Apparently the software updates from Brother are designed to look for hooky carts, and just disable the printer. A Factory Reset doesn't fix it.
Cartridge Save might be worth a try to find out how to reset the thing. I'd already used a secret code, to enable printing of the toner when it stopped at 1000 pages or whatever - so you can keep going until it really runs out. There might even be new spy info online now (not found). At the time the local Cartridge shop couldn't help either.
CS kit is cheap enough - presumably they help if you buy from them.
It's cheap enough to try again if I can get phone support, the shoulder chips have shrunk over time. Thanks I'll let you know.
Actually a link on their site took me to
https://ybtoner.com/brother/ which is USA but cheaper.