SimonM's other half Jane here. Had very similar problems with my clifford concept 300 on my GTO TT. It was fitted by Sexton's in Bath that are currently being refurbished in Aug 2004. I took it to the Bedminster branch and they looked at it for me. Apparantly the main control bit of the alarm was faulty so they replaced that. They also said to keep CDs out of the centre console where the sensor was.
I have a little ariel thing in my windscreen which must be for the remote so keep my sun visor up as that seemed to be setting it off. Sexton's said to keep any metallic objects away from the sensors. They reduced the sensitivity slightly and took the auto-arm off as I never forget to lock the car. This all seems to have worked. As a precaution I keep my little bits of chrome in the interior covered up. This is around the gear stick-looks a bit daft covered but I don't want to p*** my neighbours off anymore. My siren is in the boot as there was no room in the engine bay and it's peircingly loud.
J