ok, if anyone is interested my circuit works.
I just tried the first 2 components I got out my spares bin.
1 500 uF capacitor in parallel with a 36K resistor across the switch.
so when switched on, switch is 'closed' by capacitor until it charges (couple of secs). though switch is still 'closed' after that by resistor its at a high enough impedence for the gps to think its open (keeping the switch down would cause it to switch off after 10 secs or else I'd have just shorted switch).
when power is off, capacitor discharges through resistor. After 30 secs if power returned, it powers on again. job done.