The implication of both replies are that a timer is 'modal'. I had taken this for granted originally but lost confidence.

I may have found my problem (bug) - some testing will be needed to prove it.

I have two timers on one form. The first (and originally only) one uses an instance of a push/pop class to save and restore all settings of all areas into properties of that instance. The second was initially created by cut'npaste and stupidly uses the same instance. So, as each timer started, it overwrote the other timer's settings. The settings were not being stacked so on popping they both got the same reply. They now have their own instance.

If this cures it, then the presumed rule becomes "A timer is modal until it is interrupted by another timer" and David's description of a timer as an interrupt is a valuable one.

Thanks again,

Bill