I am able to log in to y BTinternet email on my PC but when I try to log in anywhere else using the exact same login details that let me in successfully on the PC, I am presented with an error saying my login details are incorrect.
I have attempted to change the password but this also claims the current password (which i have double checked does allow me to login via PC), is incorrect?
My email is 20+ years old and was set up for me when I was a child. I have no access to any other details from them this time regarding how it was set up
Are you presently a BT Broadband customer or are you paying for your email through a BT Premium email account?
If neither of the above your email account may have been downgraded to a BTMail Basic account which can not be used with an email client or email app such as those on mobile devices and can only be used with a web browser, which I presume you are using on your PC.
When you are logging on to your account on your PC are you entering the email address or is your browser entering the details automatically for you?
See link
https://www.bt.com/help/email/bt-email-products