Thanks for the feedback chaps.
By my earlier comments I meant that I could fill up the system so it was above the top of the pump, squeeze the top coolant pipe (as you do to get air out the system) and see the water being squeezed out of the pump through the gap between the pump seal and the head.
If sealant's out of the question, could anyone remind me the best way to get a good seal between the pump and the head? I have a feeling that if I can get a better seal then it should solve this problem but when i changed the pump I remember it being really, really hard to get that seal compressed.
Thanks
Alex
EDIT: I've just found an article which answers this exact question:
https://www.volvoclub.org.uk/faq/WaterPumpChange.html
"...using a 12-18" pry bar or screw driver, lever against the bottom-most portion of the pump and the crankshaft pulley or power steering pump and lift the pump upward. You need to get compression on that flanged o-ring." I'll give this a try tonight if I get a minute!