Thanks all for your thoughts. I can't say much until the police have finished their bit.
My suspicion is deliberate vandalism, I'm afraid. The bike is kept under a cover, padlocked to an iron grill, down a flight of steps below street level, out of sight from the road, and behind an iron gate. The steps to it are in plain view of the office, and anyone tampering with it would be immediately visible should somebody enter or exit the building.
Given the incident occurred in the middle of the day, your responsible vandal would need some sort of mechanism to ensure the office was empty before even approaching the bike. A fair amount of venom towards the owner would also help chivy things along a bit.
Its all conjecture of course... It was probably squirrels. Or badgers. Nasty things badgers.
Hmmm....