I was a bit disconcerted when the lever lfted freely in my hand, the cable came away as much as I felt like pulling it, and the engine cover didn't move 
So how to get it open?
THE SOLUTION:
I practised on my spares car which is cluttering up my garage with
- crucially - no engine, and figured it out. Basically, by inserting a long slot head screwdriver with a fairly wide blade (I actually used a knackered old chisel) into the mechanism from below, and twisting, the catch will open.The point to insert is immediately to the right
(looking forward) of the central bracket at the bottom of the lock mechanism. Insert the screwdriver into the narrow gap - 2-3 mm perhaps - between this bracket and the bit of metal immediately next to it (this is the part of the release catch which the cable pulls on, but it's kind of hard to tell that from below in torchlight and an engine in the way). I realise this is a fairly crummy description so I'll try and post a photo in the next few days if anyone's interested.It will be necessary to angle the screwdriver to the right
(again looking forwards) to get it in - ie, approach from roughly the offside wheel. Then twist it so that the sides of the blade push the release catch away from the bracket. It doesn't take much pressure, only about 5 minutes of cursing, getting foul oil all over your arm, wondering whether you're going to need an angle grinder after all, then ping! it pops open and you think Hey that was easy after all
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