My 10p's worth:
Whether the DNO's limit of responsibility lies at the meter or the switch is largely irrelevent as it would seem that the failed connections on the consumer's side of the switch almost certainly caused the incident. This is the consumer's responsibility to service and maintain.
IMHO it is quite possible that the loose connections have been simmering away for eight years, and likely because the installer failed to tighten up the terminal screws sufficiently and/or secure the tails to prevent movement. I would think sabotage is unlikely - there are easier and more reliable ways of effecting sabotage. The OP may have recourse from the original installer if installation certificates exist or via part P notification through the LABC, although a claim on the building insurance may be more fruitful.
Most significantly and thankfully, no one was injured and damage is minor.