Thanks for all the advice. I got a replacement zip from Jaycotts, it arrived quickly. I decided not to do a proper repair, ie unpick as the coat has storm flaps and zip shields and it would have taken forever. I cut the old zip teeth off and stitched the new one onto the flaps. From the outside it looks fine, there's a bit of a ridge on the inside but I'm the only one who'll see that. Bring on the bad weather!!