I should have been a bit more specific in my original
posting and called it
a "GPO Trimphone" (or "BT Trimphone"?).
I did a bit of poking around for you, and I've found the numbers of the
N-diagrams that seem to apply.
N886 is the diagram for the wired-in version, N8886 is for the plug-in
(both for DTMF models).
You can get said diagrams from
http://www.samhallas.co.uk/repository/n_diagrams/
As expected, while you get an overall schematics of the phone, there are
no internal schematics for the dialer (keypad/DTMF econder) or ringer.
May be some help, though.
-tony