You could get a pic chip, and write a little program to monitor an input line.
If the delay between the line going high again is more than a second, then sound the buzzer, otherwise do nothing.
Im just up the road from you (sunderland - small world isnt it!!!!)
I may be able to knock something up for you if you are interested, still learning pic's - but should be able to do this. If you can interface the beep output to a ttl input then I could supply the pic chip programmed to do it (you would need to hook up the pic to the beeper, and wire up a buzzer, and make sure its safe, as lots of mains in a ups.....)