"Openreach engineer installed one of their modems as a 'last' resort"
Thats a great result. The Modems do work very well to calm the line down. You are very luckly to get an openreach modem as BTRetail will be charged for that.
I don't know if TPlink has changed something or BTWholesale (BTw) has with its on going core upgrades thats seems to be causing issues.
Plusnet (btretail's sister isp) are moving to the same network type (WBMC) have an open fault ticket with BTw about reconnecting at the PPP layer. ( PPP connectivity (ref: 88851).
Well whatever it is, it isn't generic. I don't get the PPP drops on my TP Link. The firmware hasn't changed since July.