[KevinW]

Yes, you're right, I got a little distracted with the day job. But thank you all for your posts! I'll be looking into those suggestions.
My FE uses an Edelbrock dual plane Performer(I think) intake manifold, so I think its the large size thermostat?

I did try out the thermostat in hot water and it did seem to work OK prior to installation a few years a go. I then read an article telling me this was exactly the wrong thing to do, so this put some doubts in my mind.

My reason for thinking it may be faulty:

1. Top hose on my FE gets pretty hot pretty quickly;
2. Winter driving yesterday (air temps 32-39F): engine temperature only seems to get up to 70C/158F, frequently down to 65C/149F when cruising at 60mph+ in top gear (around 2.5-3k revs).

On the last point, I thought the stat should only open when the engine coolant temperature is up to normal operating point, which I have assumed is 85C/185F (well that's where it was in the summer, maybe a little more when weather reached 100F in the sun), and then should start to close if the coolant temperature fell. Happy to be corrected if my understanding is wrong.

Retracing my thoughts aloud, I guess I need to first (a) put some kind of insulated shield around the manifold temp sensor to see if cool air is effecting it, and see how warm the rad gets - wasn't enough to switch on the fans yesterday.