Fiesta 2003 electrical problem

[CW1]

Hi,
Boyfriend's car wouldn't start yesterday, wouldn't turn, no lights, absolutely dead. It was fine the day before. He's put a new battery in today but when he turned the ignition, there was a hum & nothing else. He had lights. Turned ignition off & lights stayed on ! Only way to turn them off was to disconnect the battery. He's got someone going to have a look at it tomorrow (a "specialist") but even he says he's never come across this before.
Any advice on what could be the problem ?
Thanks.

[Hymie]

My bet would be a poor earth (return path) used by the ECU. When the 0V of the electronics control circuitry rises above a volt or so, the logic is liable to go haywire, causing all sorts of spurious faults.

If you don’t have a wiring diagram for the vehicle, try to locate the ECU and inspect any close-by chassis connections (including those fixing the ECU to the chassis) for corrosion or a loose connection, causing poor electrical contact.

[CW1]

Thanks Hymie. Can't pretend a single word of that makes any sense, but least you haven't mentioned the possibility of it being the ignition system (which the garage had said would be around £500 !) :o)