I have been thinking about the sensitivity and made a couple of changes to the code, including your suggestion to just set the beepTimer in the interrupt routine and it works, I just need to decide the best way to achieve the end goal, and I think it is a combination of count and length of beep.
One last thing I would like to add is low battery detection, although this would be a bonus and is not a necessity. At 4.5v the current consumption in idle is now under 6ma, so powering from 3 AA or AAA would give a good idle life. During the on state it will probably draw less than 100ma, so battery life would very much dependant on how often it activates. Real world testing is the best way to find out I guess, once I have built the prototype.
Thanks again for the help.
Regards,
Les