Thank you, @Joseph-TP I've updated the firmware by a couple of years 2014 to 2016, lowered the peak bandwidth from "games" to "video" and moved the devices closer. Will see how this affects...

@Gerryatrix said Upgraded firmware on the repeaters to WPA4220(UK)1.2_update_1.2.9_20160525 (was 2014 V1) You never know, maybe this will help matters...

Currently using two AV500 powerline wi-fi extenders with their base unit connected to BT Smarthub via RJ45 ethernet. Mostly this setup works well. The VDSL and main wifi network is 99.999% reliable...