When I worked at Bumbleside you certainly could see the threshold of 26 (or 27 as it was) ...
It could well be that the Fire Training ground equipment now partially precludes a direct view of the threshold, but I do find it rather odd that ATC didn't see the accident occur or at least the aircraft after it had come to grief.
Niknak, the fire training ground equipment now partially obscures the view of the threshold of rw26 from the tower. As
helimutt said, the aircraft also did end up close to the fire training equipment which completely obscured it from the view from the tower. The ATCO reacted very quickly though when it didn't appear where it should on its landing roll, even though he was very busy at the time!
And just to clarify, there are CCTV cameras for the 02 and also 08 thresholds which are also difficult/impossible to see from the tower.