Just a quick update;
So after not having had much time to mess around with this car I've ordered a new expansion bottle (which came with the lower pressure cap - 11psi) to fit at a later date and in the mean time, I've drilled and tapped a pressure gauge I had lying around into the side of the existing bottle.
I let the car get up to temp while idling and the pressure increased but only in line with what you'd expect from the coolant getting hot. Then I took it for drive up the ring road and gave it some beans, when I got back to the workshop the hoses were very firm (both top and bottom) and the gauge was reading at 1bar / 14psi.
I didn't have time to take it out again to see if it would increase further but I might have some time today to give it a drive.
The pressure dies right off after the engine is left to cool down and there is no apparent lost in coolant.
I know with the original volvo 22psi cap, there's still some way to go before the car his release pressure but does anyone know if there are any figures for what pressure the cooling system should be running at?