Aaargh what is it?

Yes it is HUGE! And look at the size of its sting!

However you will be pleased to hear that that is not a sting, but its ovipositer (for laying eggs).

I looked it up and it is a giant wood wasp, harmless to people and buildings although it does lay its eggs in dead and dying trees it doesn't do it in dried and treated wood. Gave us quite a turn though as it lives up to its giant name.