Moved to the brakes sub section for you.
You say it judders under heavy braking, ...has this started just recently after some spirited driving and heavy braking? or do you think it was happening already but you've only just found out after having to do some heavy braking?
Are they new discs/pads fitted or older ones?
Just trying to narrow down what may be causing it like improper brake disc fitting (dirt on hub when new disc fitted, uneven run out or incorrect bed in procedure), distorted disc (warped ~ pretty unlikely on todays vented discs) etc etc.
Some good info here in this PDF for you: http://www.brakeworld.co.uk/pdf/tb1.pdf