Another 4 layers of abject confusion goes to Shenzhen, hopefully actually correcting a couple of mistakes found in the previous simplex version. Upon re-reading the datasheets for the amps I noticed that through holes are specified for thermal dissipation which should also make it easier to solder the chips, which seemed to me to be floating on a big bubble of solder on the central pad, not allowing the edge pads to make reliable contact. Hopefully that drama is over!

Another concern is will there be interference between the two neighbouring circuits? I guess we'll find out in about 10 days time when the boards come back to the UK.