Volvo FL10

The blue cap is the filler cap, the same type of pressure relieving cap as used on Land Rover, Commodore and no doubt other marques. 
The cap can fail, as can the plastic reservoir. Both failures can be intermittent and hard to find.

Cheers Brett, 

This header tank sits under the cab, hence why the filler sits behind the cab, to avoid tilting the cab to pre-start the coolant level. Neither cap seems to be overly pressure type, they are only plastic caps. 

There has to be a pressure cap somewhere in the system or it won't function correctly.

As long as the pressure cap seals the engine, radiator and heater it should work. All tanks or bottles after the pressure cap are just catching overflow and hopefully returning it to the radiator when it cools.

The pressure cap has to be a recirculating type or it won't draw the coolant back in.

Have come across similar issues with F12. In both cases it was to do with the lines connected to the cab heater, one of which is quite small and prone to blockage. In one case it only boiled over when the cap was done up tight like it should be - turned out to be a glob of silicone sealant floating back & forth in the header tank system. Never really got to the bottom of the other one which might have been simply that tank was over full.