engine coolant/temp gauge/heater

[redmoog]

2003 vectra 1.8sri 16v.........advice needed please
enginee coolant now needs topping up every other day, level drops 1 inch or so every time. up until a week ago topup maybe 1 every 2 months
mechanic has checked this several times, gas test on engine....dye in the header tank not leaking! he suggested the only other thing could be the head gasket (gulp) but its running sweetly so dont want to spend £500 if not required!
not sure if relveant but i'll mention it anyway temp gauge doesnt move from its lowest level (hasnt done for months), had new resistor fitted 2 weeks ago as heated only work on 4, last few days only warm air blwing out
anybody else had similar problems, if so is there an easy fix!
thanks in advance
mick

[redmoog]

forgot to mention........i have been using normal tap water to top up, is this OK