Hi Claire, and welcome to the boards!
What a fascinating post indeed.
What's interesting about that address, is that it's less than 150 yards away from SL's 6pm appointment at 43 Waldemar Avenue, which is just around the corner and is the street than runs parallel to Dorncliffe Road.
Here's the Google Street view link to the house; 10 Dorncliffe Road.
As you mention, the basement flat is accessible via the pathway to the left hand side of the building (as you look at it.)
Explore a place in a more immersive way in Google Maps.
maps.app.goo.gl
The Wormwood Scrubs connection is also particularly fascinating.
It does make me wonder if SL had at some point got out of her car to go and join the suited Mr Kipper via that side pathway?
If the sighting at Dorncliffe Road took place earlier, then it may be less relevant to her demise, but IF the sighting was later on in the day, then could SL have been attacked, drugged, and then murdered in that basement?
The killer could then have driven the Fiesta away from Dorncliffe Road and driven the car back to Stevenage Road to park it opposite the Sturgis property at 123 Stevenage Road.
I believe that if this sighting is indeed correct, that it may be a crucial piece of the puzzle.
Do you know roughly what time of the day the sighting occurred?
I also wonder whether the drugs connection may have been another aspect of the case that has been overlooked.
Recreational drug use at that time was popular amongst the "yuppies" in London; those who worked a 9 to 5 during the week, and then got high on hard drugs at the weekends.
Was SL somehow involved in all of that stuff?
What an absolutely fascinating post indeed!