Okay - I think I solved it - set it to use session instead of cookies. I thought it didn't work at first but I had several cookies set for the site and it apparently continued to read them, but once they were deleted it successfully forgets who I am each time. I would rather like to give people the option of remebering their logins (though I think many of them would naively prefer to feel that they had to enter something to get in!) but need it by default not to as they will be accessing sensitive information from insecure machines.
Jock