So, the job is complete.
Old "BT" number mapped to my Voipfone Account just after noon today.
Have rigourously checked all is as I want it, outgoing and incoming.
Have proved Ringing for my BT Phone on the local Extension Wiring I've done (As mentioned early on, the Cisco SPA112 ATA (Analogue Telephone Adapter) won't ring a BT Phone without a Ringing Capacitor across the Port, somewhere).
DECT Phones work as before. No issues.
Voicemail is brilliant: Any missed calls (I deliberately missed one from SWMBO) are recorded for access either online, or via 1571, or via a .wav sound sent by E Mail.
SWMBO is super impressed with call quality and volume.
I did try configuring both Adapter Ports identically, but of course only one Port can register at any one time with such a configuration >> This means I can ring out from either Port but only one Port "rings" on incoming.
This is not a problem (The spare Port is really to allow two separate providers to be used, not the same provider twice!) as the device has a REN capability of 3. To which end, I've spent £1.09 on an RJ11 doubler for permanent use with all instruments on a single Port. As a stop gap, I've created an effective doubler by using a (now redundant!) ADSL microfilter. Works a treat, so the household system is now as it was on copper, but without the copper.
Just got to decide whether to stick with the US Ring Tones the Adapter delivers to the old BT Phone, or whether to change them to UK ones (or something else). Personally, I'm not bothered, but SWMBO might like something different.