Logic suggests that to prevent backing up and congestion on the carriageway it would be preferable (but certainly not obligatory) to visit a petrol station on the same side of the road as you are travelling. Given the side of the road that Somerfield/Texaco is placed then this means that, if you were to enforce a one-way system, cars travelling from Marple Town Centre toward Stockport, with entry on the Marple Town Centre end and exit on the Stockport end. (Yes, this has been suggested earlier in this thread!)
That said, I don't see any of the other local petrol stations enforcing a one-way system and none of them seem to have any difficulty. A little bit of flexibility seems to reduce congestion. Moreover, there certainly is plenty of room on the forecourt to go round the pumps and exit from the end you came in.
I suggest that you don't disobey the signage for now, but ask for it to be changed so that entry and exit is permitted at the easier end, preferably with some white lines to divide traffic streams.
To be honest, it's not a great place to site a petrol station: right on a very complicated junction.