I managed to get a stainless steel heat exchanger that was rectangular and about the size of a brick on ebay (about $100). It has 20 S/S layers only1mm apart or so to exchange the heat. I mounted it under the plastic cover on the RHS guard (behind the overflow tank on a 300Tdi Defender. I guess the TD5 should also have a vacant spot there (unless you've got a Disco). I took water from the heater hoses.
Concerend about the increased risk of damage due to coolant loss, I always carry spare T-Pieces for the hoses and have fitted a a low coolant alarm (float switch) to the expansion tank, so that I get a very early warning of any coolant loss.
There's some pics of the unit being installed here :
Misc pictures by konstantinoufamily - Photobucket