Cheers guys. Yeah the kit allows you to just deadlock and it will close all the windows and sunroof -- exactly as if you'd held the button down until they'd closed. That's actually what it does, it simply clamps the signal to the alarm module to make it appear that you're holding the key in the locked position for 20 secs or so.
I haven't opened it up to have a look as it's got a "warranty void if removed" sticker on one of the screws, but I've seen photos of one and it's solid state with the usual supporting components (resistors, diodes, probably a voltage reg with some stiffening capacitors) and an 800mA fuse. There's also an LED mounted into the case that illuminates when it's active, could be useful if any problems occur. I'm pretty confident I could have built one myself for less using a 555 timer (or even a 12c508 PIC, which would be way OTT for this) but to be honest I couldn't be bothered as I really hate building stuff from stripboard and I don't really have the time anyway. There are cheap kits that occasionally come up on eBay at around a tenner, but I was happy to pay the extra with the knowledge that these are well built and reliable going by the reviews I looked up on a few of the Vectra forums. Fit it and forget it as the proverb goes.