Ah I see, I read that wrong, assumed you meant the only light that remained on was battery light, not the only one at all lol! But still, it won't start without that unless the ECU has been unlocked.
Light issue sounds like a wiring fault. Either the round plugs on the lower nearside front, by the radiator. Remove them to check for corrosion or damage. OR could be the loom from those plugs that runs under the front of the chassis. Check the main earth cable that joins the battery to the gearbox, some of them are just hanging on by one or two threads these days. Might also be worth checking the main large plug that is underneath the wiper motor, I've never seen one of them suffer water ingress though.
Light issue sounds like a wiring fault. Either the round plugs on the lower nearside front, by the radiator. Remove them to check for corrosion or damage. OR could be the loom from those plugs that runs under the front of the chassis. Check the main earth cable that joins the battery to the gearbox, some of them are just hanging on by one or two threads these days. Might also be worth checking the main large plug that is underneath the wiper motor, I've never seen one of them suffer water ingress though.