Would definitely recommend a cover - I have a scooter which I cover when it's parked.

So far (touch wood) I haven't had any of the problems described above. It's a little bit of hassle when you leave in the morning, and come back at night, but is worth it for a little peace of mind.

You can buy ones which can be chained through the wheels both front and back and which ought to deter the opportunist thief. Plus it will protect all that hardy Italian engineering from the perils of the British summer.