In the AS book .it talks about her parents wondering what to do with her flat it was not selling previous to her disappearance. And after a couple of months the parents cleared the flat of suzy's belongings it mentions the sale of the flat was to be 74000 and this only covered the banks .
AFAIK it does not state if this was including interest gathered over the months suzy was missing
It could very well be just bank interest. Mortgage rates at the beginning of 1986 were about 12%, and back then, you did not have the flexibility you have now to switch to a better rate. If we go with my guess above that SJL's mortgage was about £55,000, then after two years' payments at 12% she would still have owed over £54,000 on the mortgage.
The monthly payments at 12% would have been £579, so if as seems likely these stopped after July, after six or seven months the missed payments would have added up to about £4,000. That is, she would have owed more than than she originally borrowed. 12% mortgage rates were brutal like that.
For my own interest, I worked out what her monthly income would have looked like if she were on £16,000 (good money for a 25-year-old in 1986). Her mortgage would have been tax-deductible and her personal allowance would have been £2,335, so at 29% basic rate and the then NI structure, she'd have had just over £3,000 a year stopped, leaving her just under £1,100 a month. Deduct the mortgage of £580 and that left her under £500 a month to live on. Out of that, she'd have had to find utilities, rates, service charges etc, before paying for food, clothing etc.
The expensed car must thus have been pretty valuable to her, as must the lodger's contribution.
I think the Sturgis sign was a rendezvous point and a car phone could have been used to say the person had arrived
It's possible but car phones were very expensive and very erratic in 1986, plus they left a trail as they still do.
She was also depending on the alleged 3000 commission
The £3,000 was almost certainly just her normal sales commission. Agents typically then charged 2% of the sale price and the individual EAs got 15% of the 2%. For SJL to be due £3,000, all she needed to have done was sell about a million quid of property, which is to say about six 37SRs or so. Sturgis would get £20,000 and she'd get 15% of that.
To put it another way, we don't need to posit a mysterious non-Sturgis deal as the only possible source of this £3,000.