RENAULT SCENIC 55 PLATE?

I have a renault scenic 55 plate my wife went out in it just after the heavy snowfull the car had not been used for 2 weeks prior to the bad weather she drove for 3 miles to a shopping centre for 2 hours when she got back into the car she started the car then drove 100 yards and the car broke down she tried to start the car again but it would start then cut out. On the dashbord it said low maintenence oil change low battery it was towed to my garage and they diagnosed it being either Grank shaft sensor or Camhaft sensor they had my car for 2 weeks because they replaced these items and on each road test drive it kept breaking down the same as before .they then said it could be the battery or alternator but they same thing happened on the road test broke down again. they gave up now and said it will have to go to Renault where it is at the moment Renault have done a diagnostic test and they said they will have to strip out all the wireing which for a start will cost me £390 including diagnostic test but the cost might not stop there please can u advise me what this fault could be and where to draw the line on more costs