At least one of mine are warped, causing a bitch of a squeal. I don't really want to fork out £25 for a rotor, so my main question is are rotors interchangeable? Can I buy cheap 160mm rotors from A2Z, mtb direct or somewhere else that'll fit by Formula brakes?
yes, any 160mm will fit: i use shimano centrelock ones on my k24's at the moment.
Although have a look at trying to true your rotor first (use the calliper as a guide and then just bend it with an adjustable spanner closed over the offending bit.
in my experience brakes rub 'on-off-on-off' if the rotors are warped, and they squeal if they are contaminated or just wet. My formula's turn heads with their honking at races when they are wet but dry out if you drag them and heat them up a bit.
You may find they still squeal whatever you do to them is what i am getting at...