Yes there is a vacuum pump to suck air into the sensor, that part is constant as the sensor sits in a chamber (or at least I'm proposing to) with a feed and exhaust via the pump (12v).
There are devices out there (£3000+! 
) that do this and I've had a look at them.
I'm hoping that the amount of gas will trigger the 100ppm minimum very quickly and I'll need to use a pot to dumb it down, problem being it will dumb down the LCD result too.
The sensor is 221027608690 on ebay
Could I use a rotary switch with say 4 positions to select 4 pins on the Arduino and then use division and multiplication of the results mapped to the pins (sounds a bit caveman with rock 
)