I've confirmed the following with HR on the phone. There are basically four groups - in the order of preference they are:
The group of people who have been invited to do PAT1 - they believe that the number of people invited so far will be sufficient to fill the number of firefighter positions available.
Furthermore, there is a very small group of people who have been advised that they are suitable, and to keep fit - as they may be called in case a high number of people from the first group fail PAT1.
The third group is where people like myself are - this is the email advising that we are not required to contest PAT1 and that further information is forthcoming. Basically, it is very unlikely, but still possible to be called up (e.g. new positions open up).
Then there is also the fourth group - these are all the people who were advised that they "have not reached a satisfactory result" in the psych & abilities assessment.
The person I spoke to repeated several times that the process is extremely complex and that MFS are still crunching through the relevant data on applicants in the first 3 groups.