2002 Audi A3 carbon deposit

Hi,
Our car wouldn't start and we got it towed to the garage. They said it was the fuel pump and fixed it. They then said it needed an oil change and a new oil filter, we said ok, so they did that too. (Cost 500 Euros) We went to collect the car and the oil light came on. They said the oil looks ok, and they couldn't see a problem on the computer so we aggreed to bring it back later in the week. It got about a mile down the road and stopped. We got it towed back to the garage and they said the oil pump isn't working and it may have done damage to the engine. They had a look (another 130 euros) and said there was a carbon build up.
We live in Portugal, so they quoted us more than the car is worth (1900 euros) to fix it. The oil light never came on before they "fixed" it. Is it possible that poor quality fuel could have caused the carbon build up as well as the broken fuel pumps and oil pumps? Is it a coincidence that the fuel pump broke straight away or could it be related to their "fixing our car?" And do you think it's worth repairing and how much should it cost?
Thanks,
Rebecca