Freelander 1 1.8 Coolant - Question

So the expansion tank has the coolant level half way between min & max. Run engine at tick over for 10 minutes and coolant level rises, up to the welded join in the expansion tank. The by-pass thermostat is hot on the top side, but cold on the bottom, therefore the thermostat not opened. Coolant temperature running between 88 - 90 degrees (according to Hawkeye). No fans running (as expected - aircon was off). Temp drops from 90 back to high 80's and the by-pass thermostat is warm at the bottom (suggesting thermostat starting to open). Is the coolant level in the expansion tank rising a cause for concern or is this just the effect of the coolant getting hot? By the way having the heater setting to hot or cold (without heater fan) didn't make a difference to the coolant temperature reading. Switched off engine and half an hour later, coolant level in expansion tank back to half way between min & max..... or am I just getting paranoid???

Coolant will rise and fall a few % of the total volume, as the engine temperature rises. Excessive rising could mean there's air in the system, but if it's only raised an inch or so, then it's of no concern.

Make sure there aren't any cracks in the top of the tank, around the filler, as these will prevent the pressure from building up, which again causes excessive coolant rising.

You can force the bypass stat to open, if you hold the revs at 4000 RPM for a couple of minutes.