Going on what you said about running it low on water, I would suspect a blown h.gasket/warped head unfortunately.... sorry to say it mate

once the engine is upto temp (yeah i know your guage doesnt work) no pressure should be generated within the cooling system. once its upto temp put the cap on and leave it running for a minute or 2, remove the cap CAREFULLY and listen for a hiss, (pressure escaping). this could be a sign of a blown headgasket. (compression escaping from the combustion chamber, via the head gasket into the cooling system). This is only a rough guide mate, please dont take my word on it!!!!!!!

good luck