It has gradually got worse to the point where it has started needing water adding every 200 or so kms. When the filler cap is removed it blows off quite a bit, indicating that the system is getting pressurised from within.
After a bit of a rummage on the interweb, it looks like there are 2 likely causes....
1. The gasket has failed - quite possible because it was not skimmed when I replaced it last time.
2. The head is cracked. This seems to be a fairly common issue with the Td5, so with this in mind, will my cylinder block test kit work on a diesel engine or is the liquid change only effective for petrol engines?
One wag posted this in a forum...
A new replacement head with valves and springs is just over 1000 QuidThe normal reason for over pressursing / loss of coolant is a failed head gasket, but seeing as you've got a TD5 I'd bet my last square of toilet paper on it being a cracked cylinder head, sorry to be the bearer of bad news (if I'm right of course) but it's just the way they are. I think tyres last longer than TD5 heads! (I drive one too)

Any ideas, anybody?