My girlfriends Puegot 206 intermittently will not start. ?

In January this year the ABS ECU was replaced on my girlfriends car, a puegot 206 urban petrol model registered in 2006 because of an intermittent fault which was indicating that the abs system was not working. After waiting for over a week for the garage to fix it they used a mechanics spare unit from his car to establish that the unit needed changing! Now, at times during the past week the car is showing the airbag light when driving and intermittently will not start. After calling out the RAC yesterday when the car would not start, it started first time! This morning it will not start and the airbag light is on! We do not want to have to rely on the inadequate expertise of the local main dealers who charged us nearly £550 last time and were patently ill equipped to deal with the problem. Please can anyone suggest what we can do before we have to go back to the "experts?"