More questions!

The temp guage on the motorway the other day went to half way and stuck there nice and solid.

Just pottering round b roads for a couple of hours the day after it never got far from cold end, about 1/4 up

Day after pottering round it was at 1/2 again?

Due they tend to fluctuate a bit or do I need a new thermostat?

possibly but first check system is completely full ,by removing thermostat bung ,fill with both bungs out and refit only when overflowing same with expansion tank ,any excess left after engine is run will blow out harmlessly