The temperature sensors arrived last week, and I managed to get a suitable resistor for them yesterday, so the temperature at home is currently a cool 19.5C
Next step for them is to wire/program buttons to adjust the desired temperature and LEDs to simulate the fan and air con switching on.
Hit a snag with the 555 and the keeping of power running for 30 seconds after the ignition is off.
So I'm wondering if using a 2F or 3F capacitor would do the trick.
According to my maths, C=(t*I)/V, which would be (30*0.5)/7=2.14 (7V being the drop from 12 to 5 that the regulator uses to convert to 5V.