OK, OK, I have just figured it out... I am getting good at this php malarkey... NOT!
Whenever I inserted the session variable, it was inserting at the top of the code just below the require_once file info, but crucially above the recordset that I was trying to collect the info from... so obviously, as the session had been read before the recordset, it was empty.
So, in order to ensure that I am not carrying the incorrect session ID from one record to another once a user has inserted a reply, on the resultant detail page, I have cleared the session variable. Is this the correct procedure?
Am I tackling this in the correct manner? Is a session variable the correct method, and is clearing its value a requirement/good practice?