Although technically, they're both at fault for reversing into the carriageway, if you look at the footage carefully, the red car was actually stationary and the grey car reversed into it. I reckon it would still go 50/50 at insurer level though.
I think the red card albeit lately spotted the hazard and stopped; the grey car then continued to reverse and into the red car therefore the fault lies with the grey car as they did not spot the danger and stop/try to stop IMO.
The purpose of the car horn is to let other road users know you're there.
If the red car stopped because they could foresee the collision but failed to sound their horn to warn the grey car of their presence it is still at least partly the red car's fault.
I know this doesn't have anything to do with the claim as such but the neighbour with CCTV can pretty much point the camera anywhere she wants so i wouldn't try and use that argument if the situation did for some reason get bit more complicated. When it comes to domestic CCTV, they're usually overlooking neighbours front or back gardens anyway as the equipment often has wide angled lenses.