Just to add to my post last week, regarding max. 5Mbps download speeds on my 70Mbps Infinity 2, it was escalated to support level 2 after three days and an Openreach engineer booked for day 7, as from BT's end, everything appeared normal - 72Mbps connection and stable. But he didn't turn up (prev. job overran etc...). The phone lines between me and them became hot and I ended up speaking to a supervisor (in India) - still on day 7.
Due to Easter, she coudn't re-book the Openreach engineer until day 14 and he did indeed turn up in good time, did all the checks, eliminated my home setup and wiring and escalated it to a full "connection re-build" as connection speed was 70Mbps but throughput only 5Mbps. The Openreach modem and Homehub 3 were not at fault.
Day 16 arrived and... problem fixed! Since then I have had a courtesy call from BT to check progress and they have booked a final call back for day 21, to ensure that the connection has remained stable. We will then discuss and agree compensation.
Overall, apart from the missed appointment, I am very satisfied with the BT response and actions taken. It is far better than how Sky handled an intermittent (but many times each day) disconnection fault on my broadband a few years ago, which took Sky 4 months to sort out. So, shockingly, I can say... missed Openreach appointment apart - well done BT customer service and Openreach.