This may only apply to old UT users but it might not.
If you try to sign in and get the following error:
"You have entered an invalid username or password. Please press the back button, enter the correct details and try again. Don't forget that the password is case sensitive. Forgotten your password? Click here!"
Click the "click here" hyperlink (it should be a hyperlink), a form will come up asking for your email address. Enter the email address of your account. An email will be sent to you allowing you to reset your password. By clicking the first link in the email your account password will be reset and deatails of your account will be emailed out to you. This will include your username (which is important if you are unsure what your account name will be if you had an account on both forums).
If you still have trouble email Phil, Fraser or myself:
Phil : firstname.lastname@example.org