Just had new ABS sensor fitted to rear nearside wheel of Multipla to cure ABS message on dash, however now get judder through the brake pedal on anything other than featherlight braking. Feels like ABS kicking in every time. Any suggestions what might be wrong?

From your description, I take it you didn't fit it yourself?
I'd take it back to whoever did it and ask them to check/sort it, as the fault wasn't there beforehand.