Yes - everything seems to have stablised a bit now, I'm still using the AV500 powerline adaptors but they don't seem to be causing massive problems. I've also tried separating power/data cabling as much as reasonably possible.
The order of things is:
Master socket - HG612 - WAN socket on HH5 (with Wifi off) - Unifi WAPs
My original plan was to replace the HH5 with another router, but not found out yet if the Ubiquiti range is fully compatible with BT Multicast yet.