Hopefully my final note on this subject: I was picking up a spare case for the 'phone, so ordered a replacement top unit at the same time. This has broken the front camera in certain apps, but appears to have almost completely solved the remaining drop-outs I was getting. Pressure on the headphone plug in one direction still causes loss of one channel, but it’s extremely rare now.
So, in conclusion: my audio issues seem to have been an unfortunate combination of three factors: a bad top unit socket in my phone, the slim case being too thick around the headphone socket, and a failing plug on my headphones. If you’re having problems then test with multiple headphones in multiple devices (including with the case removed from your phone) to narrow down the cause.
[edit: nope, new top unit only lasted a month; not happy]