O.K. Finally got back to work on this today. Pulled the water pump and Thermostat housing off the block. The pump impeller looks a bit manky as it has been rebuilt a couple of times because no one here in New Zealand could supply one. I have since found you can still get them for the 4x4 over there so may look at buying a new one. Having said that I don't think it is bad enough to be causing my problems and I am taking it tomorrow to a water pump specialist to get his opinion. Also taking Radiator to be disassembled and cleaned out.
Anyway the thermostat housing has an O ring which has been not fitted properly (if I knew how to post pictures I would) and I think this would have allowed it to suck in air. Does the water pump get fed directly from the stat housing via the short hose connecting the two. If so I think this could be my problem although why it just started happenning (the pumping dry issue) I am not sure as it has been in there for a number of years. Also is this O ring all there is on the stat housing to block sealing or is there supposed to be a gasket as well. Also need to see if ford here has a pump gasket for refitting. Is this 4x4 gasket the same gasket as off any others. Will keep you posted David