Peugeot LX HDI Diesel

My car drove but every so often back fired and blew out white smoke. This happened 3 times but the third time it happened she was hard to get going again. Eventually she drove but the engine management light stayed on. I took it to a garage and was told 2 days later my car had stopped completely and the ECU unit was caput. I was told the diagnostic machine had pin-pointed 2 faulty injectors 2 & 4 and there was a problem with the positive wiring to the earth. I paid £700 to have a new ECU unit fitted and when I went to pick up the car they said I needed to fuel injectors too. As I drove home I noticed my radio code was flashing and my husband noticed on lifting the bonnett my battery had been replaced. On confronting them they said they will look for my battery and ring back. 10 days on and I am still waiting. Can you give me an honest opinion on what you think has happened. As a friend has mentioned if they tried to jump start the car they would have blown the battery and ECU. Is this possible? If you can help me I will make a donation. This is the first time I have used your site.
Linda