Right, having drained the oil and coolant to tighten the crank nut, I now find the coolant comes overflowing from the cap whilst running after being sat still for a few minutes; appears fine when moving

What is known:
Previously had overheating issue rectified by a new coolant thermometer sensor,
Radiator cap is good, new one used for a few months now,
Water pump wasn't opened during crank nut procedure, just removed,
Drained coolant was clear and as new when drained for the crank nut and again when drained today when refill attempt two was tried today when being suspicious of trapped air,
No warning light coming on where the bulb is good and the temp sensor was replaced only one or two months ago.

If there's more to add I'll add it, but any help would be more than welcome!

Bike is a 2008 MT03 (xt engine yadayada...)

Thanks