Thank you Richard, but my IE is set to delete/clear all history/cookies on exit. But, I've just cleared them again as per your suggestion.
I think the issue may be related to the time it takes for me to enter my new thread - is there a 'time out' setting on this forum? I was entering a fairly long post, with some interruptions for phone calls, bathroom breaks, etc. By the time I got around to submitting the new thread my session could have 'timed out'. If there is a time out setting, is there any way to alert the user to when this is about to happen? I am going to try to enter my post in Notepad then copy it into the Knee Post-Op form 'in one go' so as to avoid any potential time out issues. All posts I've successfully submitted have been fairly short ones, so never had the problem I've just experienced (being told I'm not logged on when I try to submit, thereby losing all data just entered).