I'm not sure locating SJL's car would have been that hard at all. As has been said, she left her purse in the door pocket, so she clearly wasn't far from the PoW.
You could tell a surprising amount about a Ford just from the key fob. I drove a new Ford in 1985, C-reg rather than SJL's B-reg, followed by another in 1988. These cars when new came with two keys. On higher spec models, the main one was what Ford called a "torch key", which had a push-button torch built in so you could find the door lock in the dark. The spare key was just a basic key, with a logo. Smaller or more basic models came with two of the basic keys. As a 1984 base Fiesta, SJL's car would have had two basic keys. MG's XR3i would have had a torch key and a basic as spare.
So you look at the basic key, you figure this is probably not the spare key of a higher model, as this is a young driver; so you're looking for a basic Ford that this key will open, and that may show signs of being driven by a woman (eg a girly umbrella on the front seat, or something). Starting with the nearest to the pub, you quickly find a Ford that has a woman's straw hat on the back seat. And the key fits. Bingo.
I struggle a bit with SJL popping out on a swift errand but then parking outside her own house. This is going to involve her in walking a quarter of a mile, there and back, so how swift will this be? Second, if the car's that far away, CV or whoever stands no chance of finding it. If anything on her property told him where she lived, the pub manager could have contacted her there. They don't know where she lives, so they would not know where to find her car to move it, and he can't search hundreds of yards in every direction. Third, if she parked outside her own house and somehow you knew this, would you actually move her car anyway? If it were me, I wouldn't be driving her car away at all. I'd leave it right there where it was. It's perfect; she went home on some errand and disappeared, and right away the lodger or one of the boyfriends is in the frame. It would be a big jump for the plod to assume that she parked outside her house to go into the pub 200 yards away. They'd more likely assume she parked outside her house to go into her house.
Incidentally, MG's XR3i probably explains the "BMW" sightings in Shorrolds. Double-parked so you can only see the roofline, you could mistake an XR3i for a 3-series BMW. Here's an XR3i:
https://www.mini2.com/cdn-cgi/image...-down//media/1986-ford-escort-xr3i.16534/full
and here's a contemporary BMW from a similar angle:
https://s1.cdn.autoevolution.com/images/gallery/BMW-3-Series-Coupe--E30--763_33.jpg
I suggest that if you were looking from a house in Shorrolds over intervening cars at the roof, pillars, door mirrors, door handles, bootlid spoiler and side glass of MG's double-parked XR3i, you could
easily mistake it for a 3-series BMW. The "sightings" of "Mr Kipper", a blonde and a BMW are, for my money, sightings of the search party: MG, SF, and MG's XR3i.