Coolant Loss 2009 TDi 2.0 VRS estate

I'm having to top up coolant reservoir with about 750ml water every 3-400 miles. Car has done approx. 78000miles. Generally running very smoothly with no evident loss of power or extra smoke/steam from exhaust. No fault codes on ecu. No evidence of simply dropping water or obvious leaks. When first happened it was usually after long hot motorway run, parking for a time then on return to car when started car ran very rough belching clouds of white smoke, then stalled. On restart rough running would clear but white emissions would continue for a couple of miles then appear to stop! Not done sniff or compression test? Any suggestions as to remedy. ? Head gasket change.

Durobin