after wasting a load of time trying to debug the undebuggable, Andrews & Arnold tech support put an end to my misery (sort of) , by putting my mind right. I was trying to work out why my DLink DSL-320B modems, which support RFC 4638 (allowing 1508 byte MRU / MTU for ethernet frames on the lan) so permitting you to enjoy a 1500 byte IP MRU / MTU payload as the payload of PPPoE on the Internet, did not seem to be doing what I wanted it to do. The DLink modems showed up on my Firebrick router UI as PPPoE MTU 1492 (=1500-(6+2)).
[I'm going to just write MTU in future when I in fact mean “MRU and/or MTU”, as my fingers are getting tired]
** So the truth is it *won't* ever deliver DSL MTU 1508 (=1500+8) if you are on BTw 20CN.
** Unless you are on 21CN or FTTx, then you are stuck with MTU 1492 (=1500-8).
[In addition to which you need the right modem, modem firmware, correct config in modem and friendly LAN-side ethernet link hardware and friendly router ethernet i/f, all of which must support baby jumbo frames for RFC4638]