Thanks for trying to help. I gave the watch a Software update last night and a hard reset. Then I ran for 40 mins this morning and covered 156.5km. (which should have recorded as just 8km).
All details on Garmin Connect suggest I ran 156.5 km. Avg moving speed on Garmin Connect suggests 251km/h.
I checked your suggestion of the Auto Lap, but all seems to be in order.