I've got a Samsung phone that I use in Aux for music, same issue. Loudest out of Left Front. But I also tried phone into a little mini speaker and also noticed no stereo. It seemed like the cable was at fault, if I bluetoothed to speaker things were fine.
I think the China cables and UK connectors are not getting along.
Then tried using Aux with my phone, on my 2010 Town and Country van. Also cable / connector problem, no stereo. Somebody told me to reverse which side of cable was in phone to get stereo back. Doing this did actually help with the speaker. I could see that the jacks on wire need proper positions in Aux port to get stereo, plugging it in and jiggling it around stereo worked, the stopped, then worked. So getting the proper hookup, took a lot of sensitivity and patience. I could tell that things were not lining up with cable, and having the Aux connector in the back didn't help. Problem has me bringing the mini bluetooth speaker along for drives.
Here is interesting look at inside of a phone jack. Guessing same goes for Aux port. Check out the pictures.
https://androidforums.com/threads/s7-headphone-issue-only-sound-in-one-ear.1008496/