Although I've not had any responses via the forum, I did receive a helpful e-mail.

For the record, I understand that I was wrong and that the ring should separate from the crystal and it is therefore possible to replace the crystal without having to replace the ring.

: I have a Carrera 73653 with a nasty crack in the crystal that won't
: polish out.

: Am I right in thinking that the internal tachy ring or rehaut is
: part of the crystal?

: Presumably this means that it's not as simple as replacing the
: crystal with something generic, I need to find a replacement
: crystal with ring?

: Would any crystal/ring from a watch with that case shape fit? e.g.
: 73353 or 1153

: Mine is a first execution blue dial with white rehaut. Does anyone
: have a spare that they would be willing to sell me?

: Thanks in advance for your help

: Andrew