I thought I'd update this in case it helps anyone.
I've still not got to the bottom of this but it didn't stop me getting an MOT so I've learned to live with it for now.
It definitely wasn't the reluctor rings and I have not codes thrown up but the esp off button doesn't do anything anymore so I think it's most likely the abs controller.
Does anyone know if the abs controller needs to be programmed or can I just throw on a 2nd hand one?