I generally fit these.
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&products_id=3423
http://www.securitywarehouse.co.uk/catalog/index.php?main_page=product_info&products_id=3846
You likely have a 20mm hole?
Many thanks for that. I wonder what the difference is between these and what I have used apart from the built in resistors. Perhaps the reed switch is of a better quality? A reed switch seems such a simple device that I would have thought that they either work or don't <G> The one on my exit door has been fine following the triggering of the other day. Could it have been a glitch in the control box perhaps? I've not checked for loose connections on the exit door circuit in there, but the box is not subject to any vibration or interference.
Cheers
Richard