In reply to jussy:
We used:
Regatta waterproof layer, over a Columbia fleece all in one, over normal clothes, over merino base layer (also very useful as a base layer at night when camping), with gloves and Stonz booties, quite often with the rain cover up on the carrier too to keep the wind off. Oh, and a balaclava!
Can find out where we got the merino from if you like, it's pricey (well, less than most of my kit!) but excellent.