Well I sorted it in the end and passed my MOT.
I bled the brakes and filled with new brake fluid, then found a quiet straight bit of road and bedded my new front pads in by accelerating to 30-40mph and braking rapidly down to 5mph and then repeat the process about 10 times. Brakes are much better now, but I reckon there's still a bit more bedding in to be done. The discs are pitted due to wear and tear, so there may be a mm or two still to bed.
Rears locking must have been because the fronts weren't bedded properly so the rears were doing their job, but the fronts weren't up to scratch, so the rears were kicking in well before the fronts - this is normal behavior.
Thanks for all of your help guys! I really appreciate it.