I had same problem with a Ford a mechanic at work said it's most likely and air lock which does prevent the water circulating.
or you may have fitted the wrong thermostat. Try  this leave the filler cap off run the enging on tick over if an airlock the water will bubble and the level drop so have some WARM  water to top it up don't forget the antifreeze if you are in the COLD UK. 
Tip: If you ever have to top up a hot engine with cold water make sure you SLOW POUR IT or wait for engine to cool down.  from works mechanic.
                
 Nobby
                
 January 2009