Re: BT engineers breaking my belongings!
Sounds like they did a shocking job, but I'm curious as to how you come up with £600 of damage?
From the way you described it you've got some brick dust to clean up, and snapped a cable (hdmi/toslink or something?)
Not excusing their poor workmanship and shocking behaviour and maybe it's just how you've worded it, but it sounds to me like they just did a crappy and messy job, and caused some minor damage rather than £600 worth.