Update 3..
Well this is strange. Firstly, I know for certain that the rubber section under the gear knob (which hides the mech- all manual cars have this) hasn't been touched. IE the dealer hasn't lifted this section whatsoever. Which I -think- you have to do, to look at/ adjust the gear linkeage.
How do I know? Well I do carpentry in a workshop, & I remember being miffed I'd got a good sprinkling of fine wood dust on this rubber area. On my new car I said to myself. It was still there when I picked car up: no way it would still be, if this rubber was lifted up, or even faffed with.
Anyway. As the gear 1/R & 2/R "crosstalk" thing I experienced I seem to recall (this idea just suddenly occured to me after I picked car up from being 'fixed' yesterday) was --possibly-- on both occasions whilst my car was on a slight forward incline.. I decided to test this. So I found a deserted side road with a slight forward incline.
I parked up, car running, put into 1st & gently pressed the accelerator, & it went into reverse. Now, as I had no-one around me (specifically no-one or car behind me) I continued pressing gently on the accelerator..... & the car went forward. Normally forward. Exactly as it should,
suggesting nothing amiss.
I tried this gentle pull-away in 2nd gear (as I mostly have a habit of doing tbh), & the very same happened.
Now: did it shoot backwards prior to going forwards? No. I'd say it went 1.5yds backwards.
Did it just go backwards whilst it was in neutral, IE momentarily & typical of it being on a slight hill? No. Definitely No.
It went "momentarily backwards for only 1.5 yds" only when I pressed the accelerator pedal & released clutch typical of any gearchange. At the exact time I pressed it. Albeit gently. At the very time my brain expects it to go forwards, & nothing but forwards. It goes backwards.
So now.................. I'm under the impression that perhaps this is -normal- functioning. That it is just a brief trait of this gearbox. You get used to it, & know that 'hey if on a slight incline this car goes briefly backwards, before goes correctly forwards'.
I don't know. I'm just guessing here. But that would seem the most logical explanation for this (albeit thankfully brief) weird facet of this car. And then for there being no evidence anyone even looked at the linkeage mech (even if they told me they did: I'm 1000% certain that no-one has lifted this gear lever rubber section over last 5 days they've had my car).
Zoot