Many thanks for the advice. I asked the question in the first place because the two options seemed quite inaccessible, but having removed the plastic cover off the top of the engine, the alternator terminal looks the easiest option.
I've just swapped my 300Tdi for a Td5 and have fitted an A bar with two (100 watt) spots. I am struggling to find a suitable power source in the engine compartment other than at the starter motor or alternator. Has anyone any suggestions?