Hi, this isn't a solution, more of a comment. I'm a 300Tdi man, not a Td5.
If the radiator has distorted that much, I agree the expansion cap should be checked/changed. Excess pressure should come out of the cap, not blow the rad.
But if the bottom half of the rad is cold, along with the return hose, and the thermostat is OK, then I'd be very worried and suspect a blockage somewhere.
If this is the first long haul, it may be the motor hasn't had to heat up fully before under your ownership. I think I'd have the whole cooling system checked first of all. I'd have thought there is the real risk of engine seizure if your carry on.
