[Closed] Why do my brakes squeal so much?

Shimano XT M775 and I'd probably rather crash than actually use them to safe being deafened.

They're on a bike which lives at my parents house so only gets used every few months. When I was up in October they sounded terrible so I baked the pads in the oven for 30 mins, after which they were perfect for a whole 5 day cycle tour. Now 2 months later I've returned, but so has the noise. Could it be to do with the cold? Or the inactivity?

I will try some copper slip but can't get my hands on any before tomorrow's ride.