How do i get my car moving again?

I have a x reg (2000) renault scenic alize, and have had too many problems in the 3 months i bought it. Firstly the car was driving fine then one day it came to a stop as though the battery had died, luckily it was just as i parked. Got a jump start but had to leave it to charge for about half hour to get it started. Got it home and the next day it was fine and it turned on with no problems. Decided to take it to Kwik Fit to test the battery which they said was completely dead and needed to be changed. After paying £50 for a new battery i was then told it was the alternator and not actually the battery. I got a new alternator from a breakers yard and had it fitted in thinking this would solve it. Picked up my car after leaving it at the garage for two weeks in the snow (was in hospital) and it started ok when i went to take it home. Decided to take it to get the inside hoovered at my local car wash. Left it to do xmas shopping fo about an hour. Came back and the people said they could not start the car or lock it. They said it seems like an immobiliser problem. Tried jumping it no joy, new battery again no joy so my only assumption is it is the immobiliser. How the hell do i fix this problem so my car can start? please help me. I have 2 young children and and due to give birth in 3 weeks so desperately need my car back on the road during the winter.