I've just started training using my heart rate monitor all the time. Previously I trained by pace and rarely bothered putting my heart rate monitor on. Since I started using it all the time I've noticed several odd readings. Some I think were caused by electrical interference as discussed in the RW heart rate anomaly thread thread. More recently I experienced several runs where my HR appeared unnaturally high in the first couple of kilometres just as described by the originial poster.
Posting now to say that focussing on ensuring that my HR monitor electrodes are properly wet when starting out seems to have addressed the problem. Thanks for your help people.