I am not too clued up on the td42, but I had a very similar issue years back on my 4.8, that resulted from the tiniest pin hole leak ever in human history!
Does the cooling system on the td42 have a bleed nipple?
The 4.8 does & even after the smallest leak the system needs to be bled, if you don't bleed the system an airlock will sit in the head & you run the risk of cooking the motor no matter how full you fill the coolant.
The bugger with leaks & especially tiny leaks is not the water you lose, but the air that takes its place in the system.
The air makes room for steam, steam that has no heat transfer(cooling) properties, equalling increased temps.
So before the spanners come out fully IF you have a system bleed nipple get the air out of the system then start eliminating other options.