Anyone "know" if having 2 different brands of tyres constitutes a fail, given all other factors regarding tread depth, condition, correct type and size etc are all ok? Please ignore the dependence on whether the tester actually notices or if you think it is a good idea or not.
Remember that it is at the discretion of the MOT tester but unless one is a cross-ply and one is a radial type tyre (which is an instant fail, regardless of condition) then there is no reason to fail two different branded tyres. There's a great guide to the motorcycle MOT here, which includes the details on tyres: Motorbike MOT Test Manual 4.1 Tyres
