She had a landline, but landlines tended to be in hallways, kitchens and living rooms. They were literally tethered to the wall. The issue with making personal calls was that if the lodger was home, he was going to overhear. SJL probably didn't want him overhearing what was going on in her love life, so if she was going to bin AL and NB was at home (or she thought he might be), then she stops off at the PoW on her way home from her parents' house and calls from the phone box.
Four-timing someone would require skilful management of time and place. While it sounds a bit unlikely that she'd blank AL all weekend then call him to ditch him when she got home, she might. She couldn't risk any of her partners turning up at Disraeli Road and running into any of the others there. So even though she was done with AL, it maybe made sense to ring him up and make absolutely sure he had got the message.
The whole point of using a payphone was simply privacy. Lets speculate (what else can we do in much of this case?) for some reason SL leaves her parents house and goes home obviously by car. She is intent on dumping AL and needs to speak to him, she is heading a party on Tuesday and does not want him around so to ensure this she must end it. She blanked him the whole weekend including returning to London without him even though he had just come back from holiday so the last bit was to finish the job. Stepping aside for a moment as to what she may have done in the lost hour between 9.15pm and 10.15pm lets put ourselves in SL shoes.
She heads back and needs to speak to him on the phone but does not know if NB is in so to ensure the deed is done and not overhead she uses a payphone, she may have pulled up to the POW but why bother? park at home then walk 200M would be easier right?
Takes her bag out the car does the deed at around 10.15pm which may have taken 10-15 minutes? drops her stuff goes home. CV finds her stuff minutes later.
The other alternative that matches NB account "could be" that she did indeed go home and simply phone from her landline because when she got there NB was out so she was alone and could make the call in private. That of course raises another issue, how did her stuff get left at the POW 200M away if she had not been there? Her making the call from home is possible but not in my mind probable.
Of course to join up the dots we need to make yet more assumptions.
What happens if the call never happened at all? we only have AL word for it? if you take the call out of the equation SL may simply have driven home parked up and walked back to the POW had a couple of drinks and dropped her stuff as she left returned home and never saw NB at all?
DV asked the right question when he said to AL did you ring her or did she ring you but I would have followed it up with, do you remember if SL used a phone box (in those days coins being entered and beeps etc were distinctive on public boxes) if he said no then it "may" have been a landline, also if it was a phone box she MUST have rung him because he would not have known where she was.
AL MUST have rung from a landline IF he rang her and also known where she was, if she rang him he must have been at home leaving the possibilities she used a payphone at the POW or her landline.
At this point what may join up the dots would be to ask to speak to the person who drove her to the party on Saturday and drove her back from windsurfing, she was a close friend she would know exactly what happened with the relationship and that may yet unlock what happened on Sunday night, SL may have told her what she was going to do that eve?
Everyone says where is the lost hour? where was she?
It might be much simpler than we all believe, the question is which bits are nailed on facts?
Leaves her parents about 9pm ish
yes
Phone call to AL 10.15pm
probably
loses stuff at POW around 10.30pm
probably (how else did they get there?)
Does not see NB
almost certain
If we assume she did not see NB there might be the simplest solution which removes the "lost hour" entirely
She leaves her parents house and drives home and parks at her flat and goes in, NB is not there.
Phones AL at 10.15pm from her landline.
Walks round to pub has a drink last orders would be 10.30pm (post dumping AL?) drops her stuff walks home goes to bed does not see NB
CV finds stuff about 10.30-10.40pm
As AL has changed his accounts I'm apprehensive to accept on face value the phone call happened it simply cannot be proved now it's just another piece of the puzzle, if it is bogus it has stuffed up the timeline for the last 36 years.
Assuming he told the police that the phone call happened it would have taken some nerve to lie about it as they could have checked I think he did speak to her but perhaps did not reveal she dumped him as this would make him look very bad. Once he had appeared in public with the lamplughs maybe he felt boxed in and could not change anything? When it was clear she was not coming back perhaps he had no choice but to stick to the narrative? maybe he was irritated with DV because he though DV may be trying to insinuate he was involved and DV touched a nerve and may be trying to get him to fess up? who knows