Esther,
This advice from Bill works. Copy and paste the https urls as instructed.
Here's how I was able to do it:
Create a mailing list password using this form:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?GETPW1=SUBED1%3DOCTEL-PUBLIC&X=&Y=
You'll receive an email with a link to click to confirm your password.
Then login to jiscmail to change to a digest subscription:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?LOGON=SUBED1=OCTEL-PUBLIC
The archives of the mailing list are here:
https://www.jiscmail.ac.uk/cgi-bin/webadmin?A0=OCTEL-PUBLIC [...]
This is an archive of the 2013 version of ocTEL.