. . . it's mentioned in one of the contemporaneously recorded police radio message logs that at 7.47am, the 999 open line from inside the kitchen to police back in the control room, or the Communications room or wherever, was requested to be closed down
At best, that would merely show that the officer who wrote that in the log thought that reference was correct. We know, however, that the BT operator isn't allowed to use the 999 system in that way, and she stated that she didn't.
She reported the line was off the hook, then later on she reported the line was engaged, and then she linked the open line from inside the farm to the police back in the control room!
At about 3:42 am, she was specifically asked why the line was engaged. She informed Pc West that the handset was off-hook. Hence the line was both off-hook and engaged at that time. She didn't at that time mention hearing a dog barking.
She was only able to do this because by the time she transferred that line to the police at 6.08am, the phone inside whf had already made a 999 call!
You've provided no proof of that - it's just your assertion, apparently based on mistakenly taking some log entries as being accurate in every particular, even though the relevant detail was explicitly denied by the BT operator.
That link via the 999 system remained in use for one hour and 39 minutes
The BT operator explicitly stated that she set up the connection to police HQ to let them listen in by using a normal line, as she wasn't allowed to use the 999 system for that purpose. The police might have suggested using the 999 system, but she was careful to state that she didn't do that. The officer who made the request might have assumed that she used the 999 system, but she didn't. Thus when the police wanted the link to be closed down, the way they described what happened could well have referred to the 999 system, even though the operator hadn't used it.
The line being off the hook, is the same as when the operator heard the dog barking!
The operator stated at 3:42am that the line was off-hook, but didn't mention hearing a dog barking until later.
She would not have been able to hear the dog barking if the phone was engaged
That's incorrect. At that time, the operator could eavesdrop on a line while it was off-hook, whether or not it was in use to make a call.
. . . a phone off the hook where the operator can hear a dog barking is not the same as a phone that is recorded as being engaged
The handset was known to be off-hook, and the handset being off-hook is sufficient to ensure that the line is engaged when anyone tries to dial it. Only operator-connected eavesdroppers (or BT technicians) can listen in on the line. The line status was still off-hook and engaged, even when the line was eavesdropped, and regardless of what was written in the logs, until the eavesdropping ceased and the handset was replaced on-hook, allowing the line to be used normally again. The police could then have taken the handset off its rest again to make a call or to restage the scene (or both).
. . . which I . . . believe to be technically possible
It's possible, and the handset being off-hook ensures that the line is engaged (or disconnected altogether).
. . . the phone was off the hook, not engaged
It's technically impossible for the handset to be taken off-hook without the line immediately becoming engaged, unless the line was disconnected (but it wasn't). Being engaged doesn't mean that an engaged tone is placed on the line. The engaged tone is instead placed on the line of anyone who attempts to make a call to the engaged number. The only exception is when BT eavesdrops on the line.
. . . she would not also be able to hear a dog barking, its not technically possible
The engaged tone is intermittent, so it wouldn't prevent the operator from listening. However, it is probably the case that the operator could choose to terminate the engaged tone she heard, and eavesdrop on the line instead. Anyone else who dialled the WHF number would still receive the engaged tone in those circumstances.
. . . unless the state of the phone altered or changed
The only change was the creation of an eavesdrop, but the handset remained off-hook and the line remained engaged due to the handset being off-hook. That didn't change until the eavesdropping ended and the handset was replaced.
. . . unless the caller had attempted to make a 999 call
Attempting to make any outgoing call from WHF would have failed unless the handset was first replaced and then lifted again so that a dial tone could be obtained.
There was clearly a 999 open line link created by the operator at 6.08am, which was requested to be closed down at 7.47am
That's incorrect, as the operator explicitly stated that the eavesdrop she set up didn't use the 999 system. The log's mention of 999 was therefore just an incorrect assumption by the officer who made the log entry.
. . that it had become engaged was noted in the log
The operator confirmed that it was still engaged, not that it had just become engaged. Off-hook is not a line status, just an explanation of why the line is engaged.
impossible, that couldn't be done back in the mid 1980's because of the old exchange system in use. There was simply no way an operator could patch a phone line with its handset off the hook, and be engaged at one and the same time, through to the police and the operator bypassing the engaged tone, to allow the police to listen in, lets say for a dog barking, or whatever, unless the original off the hook, or engaged phone had at some stage been used to either make, or attempt to make a 999 call
That's definitely incorrect. It's important that BT can eavesdrop on a line with a handset off-hook, especially when asked to do so by the police. This is independent of the 999 system.
. . . somebody who was still very much alive inside the house used that phone intending to summon ambulances for the wounded
That would be easy to do by just replacing the handset and then lifting it again, which would result in a dial tone and allow any outgoing call to be made.
. . . One ambulance to go directly to the house, and the other ambulance to remain on standby parked up in Pages Lane
How do you know that unless it's mentioned in at least one log or statement, or in court?
