Presumably something of this kind is what the police assume happened. The devil is in the detail though. How did he get to the 37SR appointment; if they left there in her car, why was it seen outside 123StR; if they left there in his, when was her car removed; why is there no sign they went inside 37SR; why did she not just end the meeting as soon as Mr. Creepy turns up; if this was Cannan, why did he use his alleged prison nickname as a false name...
The supposition I think is that she took only her purse expecting a short errand and maybe a stop to buy lunch on the way back. You'd put a purse in a door pocket while driving so it stayed put but you'd take it with you; so the purse being found in the door suggests she had only stepped out of the car for a few seconds.
As OdA has commented, women did and do carry around things like hairbrushes in handbags but SJL's was left in the office. One can over-analyse but this suggests that she wasn't going to an appointment such that her personal appearance would matter. If she was meeting a possible new bloke she would have cared about this.
The diary and being reunited with it were more important in 1986 than generally recognised. It was as important as a phone today; it had everything in it, ex-directory and work extension numbers could only be obtained personally.
But it may have been where she made notes about buyers' reactions to viewings. If she went to 37SR expecting a genuine viewing with the hitherto-unknown Mr Kipper, at minimum she'd need some way to write down all the usual details about this new sales prospect that they don't yet have about him. Does he have a place to sell? Is it under offer, has anyone been instructed? What's his budget, what's his preferred area, what's his timing, flat or house, does he need to be near a Tube, does he need a mortgage, does he have a mortgage offer, has he retained a solicitor, who are they? Is he the decision maker, or is there a Mrs Kipper?
This is all basic stuff an agent needs to know. If a supposed buyer gives dumb answers they aren't plausible. I can't figure out why she would go to a viewing not yet knowing any of this and inequipped to note it all down, or why she'd go to meet an admirer without a hairbrush or mirror.