My heart rate constantly shot up to 229bpm, despite me barely pedalling. The strap was moist, in the right place and I'd restarted the thing 2 or 3 times. At one point, if I believed the HRM 100%, I should be dead. Up went my heart rate to 229, sat there for a bit beeping crazily at me then just dropped to 0. Not gradually decreasing, just 229 -> 0bpm, like my heart had exploded (it hadn't). In the end I just gave up and copied the morning's result for the evening as it felt about the same. Not precise I know, but hey.
I'd cleaned the back of the strap in the morning after the commute in so after returning home, I cleaned it again with a wet wipe and everything is OK now. I guess there was some kind of film/residue on the sensors.