Thanks again. I've got it sorted now.
I knew the User-1 code but since I didn't know the Installer Code (the default 7890 didn't work) I had to reset everything to default values.
I had a google around and found three manuals: The "User Guide", "Installation Guide" and the "Programming...
Thanks Josh & Alarm, that's really helpful. Well it's definitely not hooked up to any phone line so I guess a remote reset is out of the question!
Sadly it's not under contract. I did call the installers for a quote to replace the sensor in the first place. But even though they installed it...
We moved into a house with a Scantronic 9751 which, as is it turned out, had a dodgy sensor. I've replaced the sensor but in the mean time opened the control box (and so triggered the tamper switch).
The yellow triangle on the control is now lit (as you'd expect). I type in the code, it shows...