Update....
Dashcam footage helped, definitely classed as not at fault, decided on the Monday after the incident on the Friday. Costs are going against the uninsured driver at the very rear (glad I had given their numberplate to the police) - my insurer have asked for the details of the RK via the DVLA (insurer was fair because they drove off, so what information I could provide I have). Middle guy by my insurer is being classed as not at fault.
Excess covered without issue by my insurance, and I have visited the bodyshop. Damage at the moment is down as a repair to the grey trim section, rest of the bumper is all okay (one minor clearcoat scratch).
Rear bumper will be removed though to check for any further damage behind it, although it looks like it has "lived to fight another day".
Overall the bodyshop were good and understood my requests for not cleaning it after I explained.
Repair starts Wednesday for 2 days
.
Need to call the Police mind as I was supposed to have a call by now for witness statements.
Edit:
Just called the police now, it is in for case review with a case worker in the next few days, so the middle guy may have got off lightly with my insurer, but the police are possibly showing interest if nothing else.