I bought a used car two months ago with a one month warranty. Within the warranty period the check engine light came on and a friend with a code reader checked the fault codes so that we had an idea what the problem was.

I took it to the garage for the check engine light problem and notified them that the rear windscreen wiper fluid wasn't coming out either.

After the service (which they didn't give me a receipt for) neither the front windscreen fluid nor the back comes out, which they said to come back another time to get fixed as they were busy, but the check engine light was off.

The next day I noticed that the check engine light no longer flashes during the ignition dashboard lights checks. I had the friend check the codes again and the same codes come up.

I believe the garage disconnected the check engine light and didn't fix the cause. How do I get them to give me a receipt for the previous work without tipping them off that I know they conned me? I plan to take this to trading standards (and use a more local garage to fix the problems, as the garage that I bought it from is over an hour away), but I figure they probably couldn't do anything without a receipt as proof.