Ok, I see where my naiveté is - C will be HUGE.
And, yes, my later solution introduced a second 'switch' - I'm thinking and typing at the same time - apologies!
I see now why the flip-flop solution is so elegant - I have drawn the attached schematic of what was described to help me understand.
I 'borrowed' the circuit from elsewhere so I'm guessing the 0.1 capacitor is 0,1uF?
I guess the current drain input R requires is not too great but to get my 30 minutes delay I'll still need quite a large R1 and C1
(I'm thinking 10Mohm and 150uF - but I'm not too sure if these are realistic or if C1 would be physically too big)
So any recommendations on R1 and C1 (and D1 and T1) would be appreciated.
Maybe the timer part is just too much.
You'll all be glad to know (or at least not totally surprised) that I'm getting the feeling I am seriously over-egging this and just put up with the switch
(or at least not bother with the timer element)