Anyone know anything about disc brakes?

New rotor / pads fitted and bedded in on Sunday. Squealed on Monday in the rain - stupidly loud.

Stripped everything off yesterday, cleaned, refitted, torqued on and lined up.

Possibly what Amey said - although how it ends up on the rotor I don't know. Possible that you wrecked your calliper and its leaking fluid. It only takes a tiny amount to escape and ruin a set of pads.

In the wet, most brakes are noisey, it’s the water vibrating between/around the pad and rotor

My brakes aren't screeching, bitches!