oooh, ok think i know what has happened here, this is an old fault that has affected a few omega diesels,
before i go any further the power sounder has been thrown ?
just to eliminate that!!!
ok looking at the battery from the top look down till you see 2 or 3 large connectors just above the abs unit, if you get your hand down there and follow the top most brake line you will probably find that it has been rubbing on the cables,eventually the insulation rubs through and the wires connect through the brake pipe, this causes a dead short through the ignition, which is probably why your ignition is still on? if you remove the battery its easier to see the cable connectors are clipped in and are easily unclipped to check the wires im on about!!
lol just read it back and it does sound a little awkward but it is understandable to me!! there was a post i did on this proabably 3 years ago now but i cant find it !!