After posting the message I found this:

http://www.medway.gov.uk/oco20030128r-7.pdf

It dates from Jan 2003, and broadly agrees with me, and calls for more
research. I suspect that nothing has been done, in line with the usual
British policy on infrastructures, e.g.

- Build the channel tunnel. When finished, start planning CTRL.
- Build Heathrow Terminal 5. When finished, start planning rail links
(Airtrack).
- Build CTRL. When finished, start planning links to Kent rail and
perhaps ordering rolling stock.

The main issue identified in the report is the fact that the Dover to
Folkstone tunnel can't take modern trains. This would therefore need
to be maintained by a local service, say Folkstone to Ramsgate.
(Currently a 48 minute journey for some reason)