Just gone up to a 180 from 160 rotor on the front, and now under braking the fork judders. Anything I can do to stop this?

Brake less

Check everything at the caliper/rotor is tight, check the pads, check the rotor is straight, make sure you have the right adaptor, check forks for play and check headset.

? contaminated pads or rotor?

Check everything to do with the caliper is sqaure and aligned correctly, then just ride for a little bit, it may be the disc needs to bed in to the pads and vice versa.

also not unknown for it to be caused by wavey or non-concentric braking surfaces, or pads overlapping the braking surface and catching on the rotor's spokes.