I went to Ikea in Southampton with the wife and despite the tedium of endless Swedish flatpack, I came away with a large white long-haired sheepskin for £22 which is a perfect fit for the bike (presumably its normally used as a rug).
They also do a grey short-haired version for £15 which probably looks more suitable on the bike, but I wanted the 'deep pile' comfort for my deep piles.
It makes the bike look like a polar bear has died whilst riding pillion, but for £22 it should be nice in the winter and for long distance riding.