Whos fault is it

The person who broke it should pay.

Who else?

A tough one.

It should have been agreed (due to known fragility and what is a fairly non-standard job), not to do the sockets, unless willing to carry cost of damage.

Other option to instruct sparks to finger tighten sockets and let mirror installer tighten.

nor sure what your trying to say

yes the spark broke it but if it was installed better then it wouldnt have broke.

Forget claiming from his insurance, that's his job. Footing a 700 euro mistake is crazy in my opinion. Why would you imply to the electrician that he doesn't have to pay?

Unless you are inclined to think he has completed over 700 worth of labor/materials in your house, not paying him isn't going to help you much. As long as both contractors are registered and work is invoiced along with receipts then I would go down the small claims route if communication fails. It is a simple process and costs only 25.

if someone hung a picture on a wall i a really delicate way and someone brushed off it and it fell and broke whos more in the wrong , the person who tipped it or the person who created the sitation where it could be knock off with a little tip.

leaveing the glass unsupported in an obviously very weak position is createing a potential for breakage . the spark is wrong for overtightening but the glazer created the problem

Why was it that the glazer thought he needed to say it 3 times? Because it was so fragile that nothing can put pressure on it, or because the electrician looked like he was going to use an impact gun or something? Sounds like the socket needed to be left too loose rather than not over tightened. Plugging things in might have cracked it eventually. Whose fault then, the person plugging the item in?

Electrician should have asked glazer to set the screw torque to delicate glass install setting himself once he mentioned overtightening.