Raymarine E80: Strange Behaviour

[nigel1]

I was sailing back from North Wales to Liverpool this last Monday, and approaching the Rock Channel which is the short cut into the Mersey, steering a course of 080 True, and had been making about 083 over the ground when I noticed that the B&G Network Nav at the helm was indicating a COG of 020. Watched it for a couple of minutes and it continued to indicate this COG. Bit odd thinks I, I can see with eyeball that I'm going in the right direction.
Went below to check the E80 chart plotter and it shows the boat in the Burbo Bank wind farm, about 1 mile north of where I actually am.
No alarm condition on the E80, and the fix quality was good. I turned on the back up Furuno GPS, and that showed the correct position.
Turning the E80 off then back on resolved the issue.
Has anyone experienced something similar, or has an idea of the cause.