If you are a CTC/British Cycling/LCC member then call them up and talk to their lawyers. Even if they only help you follow the usual routes it's good to get input.
If you aren't a member, consider becoming one! Then check your home insurance, it may include legal cover.
So, it seems royal mail have a specific address for reporting vehicle incidents. Not sure if I should do that or if I should go directly for a lawyer.
That bike had huge sentimental value (plus a lot of actual value) and my long weekend away cycling just got stuffed.