Thanks Oliver. Just seen this and thought would update in case the details are useful to others.
The passenger, who was initially helpful, has now gone quiet and refused to give the driver's details. That leaves me without a registration as I didn't get that detail before the driver left the scene; neither did the witness. I've been in contact with the Ally Pally office to get hold of the CCTV. They weren't particularly helpful to start with so I have submitted a Freedom of Information request and they have until 12/6 to respond.
I've checked with my home insurer and as this bike isn't one of the named items on my policy it is covered only up to £500 (less £250 excess and loss of no-claims discount).
Police have come back with formal notification that without reg details they will not be able to take the case forward. However there is a 6 month window to update in case the reg plates are discovered.
In the meantime I have appointed Leigh Day to act on my behalf (covered by my gold BC membership). If a reg plate comes through then they will proceed with a claim against the driver's insurance. Otherwise we will be submitting a claim to the Motor Insurers' Bureau - who I understand are an 'insurer of last resort' and cover such incidents. Hopefully that will cover some of the £800 I am out of pocket...