When I get back to the UK I think I will get an iPhone anyway and just add a MP-3 count-up recording. That way, however long I wish to develop for I will be fine. Stop and fix times are far less critical and I can just add five mins from wherever I am when the stop if done. This should be ideal and ensure that even if I have a 20 min development time, I will have reminders every 30 seconds etc. Most timers only seem to give a voice output on the countdown function and at pre-set intervals which are not ideal i.e. one only gives intervals every minute at below 10 minutes.
The advantage to count up will be if I am running two processes i.e. N development and N+ I can put the N+ in first, the N batch X minutes later and then just know the number of minutes at which it all comes out into the stop.