This is mentioned only in AS. All he says is that, mystifyingly, the car was seen even earlier than 12.45 - by a schoolboy, at noon.
The garage owner gives the most reliable sighting, in that as you say, he noticed the Fiesta since because of it, he could barely get into his garage. He was sure of the time because he'd arrived home from work.
I've thought about this quite a bit. There was a huge number of Fiesta colours, especially if you go back to the original Fiesta of 1976 (if you saw the top half of a Mk I from the side, it would be mistakable for a Mk II quite easily, as it's the bonnet and wheels that are different). A Fiesta enthusiasts' website I've found lists 118 different colours or two-tone combinations.
It's impossible now to know what were the commonest colours, but you can filter out those that were only in the range for a a year or two, i.e. they were special editions with a unique colour for marketing purposes. On that basis, the most consistently-available colours were
- Diamond White
- Strato Silver
- Black
- Venetian Red
- Cosmos Blue
- Ivory White
- Midnight Blue
- Nimbus Grey
So three of these eight are either white or could look white (Strato Silver). As well as these, there was Sierra Beige, Platinum, Cordoba Beige, Dove Grey, Crystal Green, Pastel Grey, Ceramic Blue, and Cameo Beige. These are all light colours that you might mistake for - or misremember as - white.
ChatGPT reckons Ford sold 1.76 million Fiestas between 1976 and 1986. The commonest colours seem likely to have been white, silver, black or red.
Two different white Fiestas, or one Diamond White (SJL's) and one Ivory White, or Strato Silver, or Platinum, or Cameo Beige one (the 12 noon Fiesta) does not seem out of the question. If it were two Maserati Boras you'd assume otherwise.
This is why it's so frustrating if the lead about a LHD BMW came in at the time. Finding a random LHD BMW in 1986 should have been doable because it's not like there are 1.75 million of those to sift through.