Shirley Banks' car was indeed a rust bucket, HWL 507N, so 13 or 14 years old. IIRC it was worth about £100 or £125. Cannan repainted it blue himself; my guess would be that, rather than SLP386S being some veiled reference to SJL, he just changed each character randomly.
I struggle with why he would leave clues to other crimes (or for that matter use his supposed - actually his later - prison nickname as an alias). The type of clues Cannan left were things like carrying the tax disc around or leaving DNA samples with the victims of his rapes, i.e. they evidence his deep stupidity more than his cunning. He has never been charged with three murders anyway.
There were also MSM reports of a red Ford Sierra that allegedly JC had access to at the time (1986) that displayed a VRM of SLP386 (no suffix). It was allegedly tracked down to a scrap yard in London by police where forensics identified a DNA trace of SJL. So much has been written about this investigation by MSM, much of it inaccurate and poor reporting, so there are key elements that would be very relevant to us on WS, if corroborated fact, rather than column filler to sell papers.
I suspect that JC's narcissistic view of himself is that he is superior to everyone and certainly smarter than the police. Would he tease and play games with them, feed them red herrings (kippers) via third parties who he has mentioned 'something' to?
In amongst all this his self-assurance of being so 'great', whilst not being that smart ('sedimentary period, Macchu Pucchu' etc) he will have made errors that were not detected and may still be. His MO is not one that involves detailed planning but one of poor emotional control and taking significant risk of being identified/arrested, which was the case, even in the days before all the investigative tools of today.
The certainty with which the police pronounced JC as the key suspect surely indicates that there is credible information. I would not be surprised if SLP's personal diary was the source of this.
My thoughts are that the mystery man that SJL was involved with, who was pestering her, is identified as most likely being JC from the circumstantial evidence of his MO, locality, some eyewitness sightings (unreliable) but his identity is consolidated by the entries in SJL's personal diary.
SJL worked on Saturday 26th June 1986. Is it this the day that she wrote the Mr Kipper entry in the work diary ....it has been reported as such but......accurate? My feeling is that SJL answered a phone call from JC in the office that Saturday, with him wanting to meet her. She was busy for the rest of the weekend (Worthing on the Sunday) but wanted to stop seeing him, so arranged to meet on Monday around 12:45.....she wanted it done. Meeting JC and getting him off her back took priority over her 'mislaid' diary etc at the PoW, which only became apparent on the morning of late Sunday evening/Monday morning (27th/28th July).
The location of 37 Shorrolds Road in the diary may or may not be correct. There is some witness evidence that places a similar couple, bottle of champagne etc in the area....I always doubt this unless it's independently corroborated....people are so open to suggestion and wanting/needing to help!
I have a thought that the diary, cheque book and postcard may have been 'borrowed' from SJL at Worthing, by a 'third party', who was trying to identify her previous movements or who she had been meeting. Could they have been 'left' in a convenient location outside the PoW late on the Sunday evening after the customers were seen to leave, for the Landlord to find and seek to return them? This person has obviously been eliminated of any involvement in SJL's disappearance.
Anyway, if it was JC that SJL met on the Monday afternoon, I think that he may have already been forming a significant degree of resentment towards her, if he had sensed her lack of reciprocity and maybe antagonism towards him. Her ultimate rejection of him was the switch sealed her fate or maybe he had already decided the outcome!
JMO