Cha - I had mine done earlier this week. They've made a little flap in the carpet under the right seat in the very back. The garage chap did tell me exactly how it worked, but my eyes glazed over in seconds. I think a plastic one is fine as (as far as I recall) it's pretty high up and perhaps can only be got at from inside the car. I think I'm right in saying that it just means that you need to release the spare wheel thingy from inside the car and it would be difficult (or v difficult, perhaps) to nick it from the outside without first doing the release mechanism from inside the car. The garage also put a sticker on the back bumper bit next to where we have the towbar saying "Warning Spare Wheel Security System Fitted Vauxhall" to deter would be thieves. Hth.
Thanks again, Yorkiegirl!