I see in some the coachline/side trim combo on some/ the rectangular number plate on some/ but does seem a more angular stumpy sort of shape.
I wonder if the rectangular number plate could be a false assumption. I think our best chance for getting close would be a psychological profiling of Gibbo's mate as was in the 1980's. Gibbo did say he had a Vauxhall chevette company car. Move on a couple of years and who knows he might have upgraded to an Astra, and the general shape and the double "trim" lines down the side could fit.
I think Gibbo could probably confirm whether his mate would buy a yugo, or maybe even a Japanese Car. I think we can rule out French, I had a brief flirtation with the Alfasud but rejected it, German? possibly an early German made Opel Kadett ( the Astra Equivalent one) but nothing else remotely like it. In the end I keep coming back to a British Marque....80's man from the Midlands...is he going to buy Japanese, when he gets a "free" Vauxhall from work. Would they be a two car family?
It would be handy if Gibbo knew the answer, but I wouldnt rule a two door Mk1 Astra, or if Gibbo's mate turned Japanese it may have been something like this

Nissan Cherry in Autumn 1981Charles01 / Public domain
REgards Neil