Just wanted to post an update on the situation. The car has been with a local Ford garage for a couple of days and the technician advised me that the error code was actually P0420, catalytic converter not operating efficiently (or words to that effect). The car has had a new catalytic converter fitted and the engine management light is no more. The technician also advised me that if the problem really was due to the voltage supply from the battery or alternator then the battery light would have illuminated on the dash, not sure why i didn't realise that!
The moral of the story is not to rely on self-diagnosis from the DTC fault codes provided by the dash, get the error read properly with a fault code reader before guessing what is wrong and splashing out on new parts that don't need to be replaced!
Thanks for everyone's help.