I am sure this is a simple circuit but it been a while since Ive been
building circuits.

I have a circuit that is controll by a PIR (not important) where the circuit
is NC (12v)

When the circuit become open I want to activate a relay to switch a 240
supply. This relay needs to remain activated for a fixed period (say 10mins,
either fixed in the design or adjustable - not important) and then switch
off, even though the 12v input circuit will become closed after say 5
seconds - i.e the circuit holds the activation circuit.

Now for the complicated part!....... I dont want the 240v relay to hold on
until the time ends and then switch off and start again (as it would cause a
break in the 240 circuit), but to hold the circuit open for the time (say
10mins) after the last break in the 12v circuit.

i.e say the 12v circuit breaks, the 240v supply is activated by relay, the
12v circuit closes, five mins lates there is a break in the 12v circuit for
5 seconds, five mins later there is a second 5 second break in the 12v
circuit, then no further break occurs so the 240 relay closes after a
further 10 mins.

i.e after each break on the 12v circuit the 10 min timer starts, ever time
the circuit "activates" the time starts again.

Can anyone suggest the simplest way to acheive this?