I've finally figured out why this forum would not allow me to log in without having to go through the "reset your password" routine.
When I signed up here I entered my name. Frank Cox.
I just noticed that when I am actually logged in, my name at the top right of the screen is shown as FrankCox.
So what happened here is that when I signed up initially the forum software happily accepted the space in my name but then silently ate it when writing the record.
Logging in as FrankCox seems to work fine.
The moral of the story is that this forum software doesn't allow spaces in usernames but it doesn't enforce that restriction when you first sign up.