Thank you everyone for your kind suggestions. I wish I had signed up to this forum for some advice prior to embarking on this fiasco...!
As the bike is now gone I have some 850gs gear to sell which maybe someone here might be interested in. Both of these are brand new:
1. Touratech Crash Bars (unused)
2. Normal height seat (unused)
If anyone is interested please PM me. I can arrange shipping.
By way of an update, I've not yet been able to resolve everything with insurance. Despite the fact that the bike was taken from a public road over a few miles from where it was registered to be kept overnight, the insurer is asking for proof that I normally keep it in a Garage. I can provide this but not at the moment due to the fact that the garage is located within a business carpark that is now closed thanks to COVID. They are refusing to budge on this requirement. It makes no sense to me! I could see how they would ask the question if it was taken close to the regular place of being kept (i.e. "why didn't you put it in your garage that night?") but whether or not it was normally stored in a garage overnight is irrelevant if it is taken from a different location miles away. The point of bikes is to travel, it's to move around. They are being difficult.
I am also unable to fulfill the requirement they have to share my driving licence code with them using DVLA's portal. This is probably because DVLA looks up the name of the driver against an HMRC database using your NI - I have a long name which doesn't get registered the right way very often, so the online portal never works for me. I've called DVLA and emailed them but, again, due to COVID, they are not answering their phones or emails. The whole of DVLA! Madness. I've told the insurer and they have told me that I simply have to wait for DVLA to go back to work. When the hell will that be?
The other problem I have is that they are forcing me to pay the £2k excess, even though this is not an accident and I am not at 'fault'. I guess it's because the police didn't find the guys who damaged the bike. What this means is that even if I do claim I will be set back for £500 for the policy, £2k for the excess, £120 for BMW's estimate for repairs (which the Insurer ended up not covering) plus the loss of the costs invested in the gear above, new levers, etc. Cost to repair the bike according to BMW would be £3.5k so it's still less than that, but this is crazy.
Long and short of it, I spent close to £10k on a 850gs in May and will not see that money back. I might see £7k back sometime in October when DVLA re-opens. Maybe.