Hi Emma,
Im pretty sure he was telling you the truth that nothing showed up on the computer because they would not have the softwear to read your car. What he should have told you in the first place was to take it to a citroen dealer to get it checked, but this would not pay him!!
In my experience advance pitstop are ok for poor quality ''over priced'' exhausts & thats about it!!!
My advice is phone couple of citroen dealers & get price to put car on computer for you. Im sorry to hear you have bein ripped off before & completly understand you having reservations.
When you know exactly what the problem is price some local (non dealer) Mechanics to fix it for you.
Keep in mind if you need parts depending on what it is of course you could try scrap yards, most do next day delivery & for new parts its hard to beat Micks garage.ie. they have a huge stock, well cheaper than dealer prices & top quality also next day delivery. I have no interest in this company im just a satisifed customer.
Id imagine you will have to make appointment with the Cicroen garage you will not just be able to drop in with them.
Hope this helps