How difficult to replace ecu?

2 weeks ago my car refused to start. it was running perfectly up until then so i thought it was something simple like glow plugs because of the cold weather. i had it towed away and it turns out it's the ecu.i don't know exactly what's wrong but i'm having this replaced. i had to wait ages for the new ecu to come and now they are saying there's something wrong because the new ecu isn't accepting the codes. now they have to get another new ecu as they believe this new one is faulty. i'm just wondering how difficult it is to replace these parts and programme them to the car (it's a diesel peugeot 206). i've only had this car about 5 months and i really like it, now i'm thinking i'm never going to get it back. are they likely to tell me the car is not repairable? or is everything repairable? many thanks if anyone can shed any light on the workings of ecu's there's not much general info on the net.