3 ways of doing this (well several actually)
1st as frank suggested non nats ecu and loom
2nd if you have a complete k11 at your disposal use the bits from that, to get round the nats either,
use all the bits from your car and get the key and tape it to the nats pickup ring and hide it away...
most motorfactors can get exhaust tubing, made by either bosal or walker, fairly cheap to buy, but you will have to buy 2 or 3 meters,
if you goto exhaust places they might let you rifle through there scrap bin for free or at worst a couple of quid
using your rough figures
£10 petrol at £1.40 per litre 7.142 litres or 1.58 gallons
40 miles on 1.58 gallons is 25MPG
1 litre auto i wouldnt expect much more than low 30's
so with these cold mornings letting the car tickover to warm up? 25 might be a bit low but might be about right?
well thats not going to stop the lights is it, and flattening his battery,
fill the engine to the top with oil and it wont stop the light coming on while the engines not running owing to it being a PRESSURE switch and not a LEVEL switch
same with battery light, it shows the battery is not being...
i suspect a dodgy ignition switch, sounds like the instrument cluster is still getting +12v after its turned off,
either that or someones been tinkering with the wiring and made a hash of it