how do i start my car if it says immobiliser fault?

I have had this car 2 months it is a renault megane expression, 53 plate 1870cc diesel.
i have had a problem with tyre sensors saying tyre pressure needs checking, so i pumped them up and one of the sensors snapped off the tyre and went flat, i put the spare on and took it to my garage. they have replaced the sensor and changed the back brake pads and topped up the oil, as the car started saying it had a parking break fault, then oil level low.
They have now said they cannot reset the sensor on the tyre and the car is saying it has other faults including the battery they said. They advised to take it to a renault dealer to get the sensors reset and to get a diagnosis of electricqal faults as it seems to have many.
The dash car image is now showing a wheel missing but the wrong one to the one that broke. The next day i went to go out in the car and its saying i have an immobiliser fault and will not even start. It seems to be going from bad to worse. i have booked it in with the dealers in 3 days time but i cant even start the damn car now to get it there.

c barrass
December 2010