I’ve taken the pads out, checked the pistons (fully retract when you push them in), checked the rotor (doesn’t seem to be warped) and lined it up nicely so it doesn’t squeek in the garage when I spin the cranks. But after about 2 minutes of actual riding I get a squeek on every pedal stroke/revolution of the disc. Is this a marginally warped rotor that isn’t really visable to the eye, sticky piston or what? It’s doing my head in.

Bit of flex in the frame/fork allowing rubbage?

It’s the back ones – They’re fitted to a Patriot with a maxle rear axle so how much flex could there be?