Hi mate, from my experience, replace the thermostat, which is a cheap quick fix, make sure you bleed the system as airlocks are common and have silicon gasket sealent available....also check the wire on the engine temp sensor on the right behind the dizzy. There is one for the guage (think its the top one from memory) which can become corroded so get rid of any corrosion on the connector on it....If you need to replace it they cost about £7. If nothing changes and your guage still goes to zilch, then its probably your gauge itself. Try giving it a mild tap on the back to nurse it back to life (not too heavy!) - if It springs back to normal temp its likely a dry solder joint - so either re-solder if you got the skills, or get yourself a new guage unit on ebay/scrappy. Those are the three common causes in my experience, all of which Ive replaced over the years.