Temperature guage fluctuating

Since fitting a new head to my rover 200 (1.4 k series) a year ago the temperature guage has always tended to read a bit low and variable. However this evening after a 2 hour motorway drive it suddenly started fluctuating quite rapidly (period of a few seconds) between normal and a very low reading. On slowing down to 50 it settled back at normal, but on accelerating it started fluctuating again.

Coolant level is fine, there is pressure in the header tank, and I checked the bleed screw, no sign of an airlock there.

However the pipe from the inlet manifold bleed all the way to the header tank, and the header tank itself, was stone cold. Could it be that the ball bearing valve in the inlet manifold is stuck resulting in an airlock in the head?

If so, do I need to remove the inlet manifold to check/free it or is it sufficient to take the hose off and poke it with something?

Or is it likely to be something else - eg the thermostat?

The connections to the two temperature sensors seem to be ok.

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