Bike had it's first MOT yesterday

Passed with just 1 advisory - apparently I have a slightly warped disc on the front.

How did I manage to warp a disc?
How easy are they to change?

Common is disc warping, with many bikes so it is nothing to worry about!

Easy to replace, get the front wheel of the ground, remove the front wheel and undo the five disc bolts, replace the disc, replace the bolts (need torquing), and pop the wheel back in taking extra care with the speedo assembly.

Do you use your front brakes hard? If not, then maybe the pads are dragging on that side heating the disc up and causing the warping. A caliper service might be in order and possibly new pads too.

I've never been a heavy braker in/on any vehicle - I'll have a look at it in the week and see whats what.

[yorkie_chris]

Dangerous dave is right... however... when you take the wheel off, rest it on a tyre or on some blocks of wood so the disc on tother side isn't touching the floor.