Dave
I think if someone wants your motor, they will take it. A couple of hours with a spanner later, and it's all in nice little unmarked pieces read to sell on...so one solution to this is to security mark all the pieces.
That said, opportunists can be deterred, and I don't think there's much better than a bloomin' great big yellow thing you can see from miles away. The only problem I find with my Disklok, is that's a big beast of a thing to find somewhere handy to put in once you've taken it off.
I've also got a 'Clutch Claw'
http://www.centinel.org.uk/clutch_claw.asp but I think on balance there are better pedal boxes options now. Have a look at X-Eng - that folds flat I think so there's nothing to store when driving.
There are also GPS trackers you can get which obviously don't prevent anyone nicking your car, but can help the Police get it back for you. Have a look at Skytag, although they are apparently friends with the Police, it's a subscription service...
The Centinel guys that sell the Cluch Claw also sell posts to put in your driveway....
Then there are alarms and what-not and individual electrical or mechanical solutions that some people like to devise....
I have heard of bonnets being nicked, and know there are a few security hinge sets out there. I think Brendan may sell some, or X-Eng too.
I also think the basics of thinking about where you are parking, what's left on show in the vehicle, turning the wheels etc shouldn't be forgotten either.
I'd start with a Disklok and then move on from there once I'd found some more money or saw what was going to make a difference to my insurance premium.
Cheers
Mike