If you are loosing coolant it could be the head gasket. forgive me if you know but there are various way a gasket can fail. As Lynall said one is for the compression gasses to escape the cylinder & get into the water system causing pressurisation (like mine had), The coolant could get into the cylinder but not escape into the water or oil galleries, this often causes some light coloured smoke when running. The other ways are for oil to get into the water & visa versa. If coolant is getting into the oil, the oil often emulsifies and looks "creamy" , you can sometimes see this on the underside of the oil filler cap. If the oil is getting into the water then you will see it in the expansion tank.
If coolant loss is your issue and you have eradicated all obvious leaks then i would be keeping an eye out for almost steam coming from the exhaust.
Sadly the TD5 engine is a little prone to head gasket/head issues so may be on its way...
I bought my TD5 recently knowing it had a issue, where mine was pressurising it had caused numerous leaks as the pressure was simply too much for the hose clips etc, the underlying issue wasn't coolant external leaks but the gasket failure.
Sorry if you know all that, good luck with getting it sorted.
