I am trying to set up automatic forwarding on messages on one of my sub-accounts but cannot find details of any of my sub-accounts - I have two.
I also don't understand what you mean when you say you can not find details of your "sub-accounts". Are you unable to log onto the email accounts?
If you are the BT Broadband account holder you will have been given the option to set up eleven email addresses.
To check if the emails are still functioning and available, assuming that you set up email accounts/addresses and did not "gift" the email addresses to another person you should be able to "see" these email accounts via your MyBT by logging onto your MyBT and going to "Your Products" then "Email, manage".
If the email accounts are not there it means they are not associated to your BT Broadband account.
To set up "forwarding" you just need to log onto the email account that you want to set it up on. Once logged on you go to the settings found by clicking on your username at he top right hand side. Then go to "Mail" > Auto-forward > and fill out the relevant sections then click "Save".