Step Turn - Nope... There was a faded number/letter which was illegible on the top side of the wing in red. I assumed that was the old sticker stating AVGAS. Fuel selector didn't have any real markings, just a faint engraving of Left, Right and Both. Perhaps you are right - perhaps it wasn't airworthy! I have heard that this particular plane was flown accidentally without CofA for 2 days because nobody else checked the documents. I did have a look at all documents I could get my hands on (especially after landing) but was unable to find anything re: fuel tank types.
TangoAlphad - it was her bigger sister... 172! Sounds like you made the right call!! Scary how both the too much fuel, and the not enough fuel situation is a potential killer...
I completely agree with you - had you, or I crashed as a result of the information given to us, something tells me that this would be put down to pilot error, and not implicate any third party despite it clearly being a combination of faults. Neither you, nor I had any accessible means in knowing this information, so in my mind the instruction on check-out is at fault!
(I heard stories about solo flights during training ending in disaster due to incorrect instruction - including one at Biggin where the instructor didn't drain the water from the fuel tank before flight, nor did the student, and I believe that the student pilot ended up in a tree just off the runway at Biggin. Not sure what the implications to the instructor were, but I doubt it was as severe as was deserved!)