I read it while looking for a solution on the Mazda Forums, a few people with the same error code stated that it was a legal requirement, so I didn't read anything official that said it was.
It's not the only issue with the car, but in my eyes it was the most serious as from all the information I'd read I was under the impression i'd end up needing to spend a lot of money if it was an MOT failure and was worried I was driving around a car on a dodgy MOT, I would have only considered Rejection if i'd been mislead
But these responses have opened my eyes a bit so I'm a bit clearer / wis