You could do it quite easily "due to circumstances beyond our control we can't bring you GMB at the moment. We're trying to get things back to normal as quickly as we can, but in the meantime..."
Incidentally is anybody else a little surprised that the fire brigade announced it as being a fire in the post room rather than simply an office?
That makes it very easy to jump to conclusions about what may have caused the fire, which I would have thought they would wish to avoid
Write that down in your copybook now.