Problem solved! After a concerted process of elimination (i.e a bit of a fiddle about!) it looks like the issue was a mix of input levels and ignorance 😁
The AUX input doesn't seem to work with an iPhone/Airport Express feed even with the levels adjusted by the dials underneath, however, works fine with the TAPE input.
I've also switched the PHONO input switch and the volume is much improved, I think this may have been because the previous owner was using a non-B&O turntable.
I also soldered up a new set of speaker cables and checked with a multimeter just in case!
Thanks for all the suggestions, glad I've got up and running!
