I've now stripped it, cleaned off all the muck, smartened-up the Bakelite with the usual Greygate polish and gently cleaned the dial's three contacts with a piece of paper moistened with Servisol Super 10.
Then I stuck it, as-is, across the line via the master socket's ADSL filter... Who needs a Rotatone, mic. inset 21A and "modern BT plug conversion"?!
It works perfectly in every respect. It dials out reliably, the bell sounds wonderful, the somewhat rusty receiver is loud and clear, and even the original carbon transmitter works as well as it should! Not bad for 60+ years old.
Assuming it's REN isn't too high, I'm now going to see if I can find room for it in the house somewhere
Nick