In article ,
Richard writes:

If not has some enterprising soul worked out a circuit that will mimic
the operation of an intermediate switch, perhaps with suitable relays,
that can be controlled with an ordinary pull switch?

Can you change the layout of the circuit so the pull-cord is one
of the two way switches rather than an intermediate switch?

Another option would be an impulse relay, and changing all the
switches to momentary operating types. This is how multi-way
switching is done in some other countries (e.g. France), but
I don't know how easy it is to buy impulse relays in the UK.
(They change state each time they are operated, and are available
in DIN rail mounting form to go in a consumer unit.)

--
Andrew Gabriel