Eventually found that it is a U.FL connector. The only source I can find is at
http://www.elfa.se/elfa/produkter/en/5835.htm
Unfortunately, it only comes with a right angle connector which is impossible to fit in to the hole above the surface mounted connector on the card. I had to take off the plastic cover, fit the pigtail and then cut a small notch in the cover to feed the coax through. The cover keeps the pigtail firmly in place. The cable is only 1.3mm dia so it's a bit fiddly to solder a connector on to the other end but I used an SMA male so there's not too much strain on the pigtail. Results are great-see pics of improvement with external 3dB omni.