We use cold water and we havent cleaned the broken pane since it was broken. The break happened between July and Augusts clean. I know for a fact that the window was fine on July's clean and when we did Aug clean there was a circular crack on the pane so I didnt clean that panel. Its the bottom pane of a sash. This month matt was cleaning the top pane and the disc fell out of the hole. The customer cane out and said that her husband was supposed to have rung him months ago.
That's significant
How many people, believing their window cleaner had broken a pane would dither for 'months' before tackling him about it? however that crack was caused, it wasn't the end of a pole, there would have been an impact mark (probably small hole with cracks radiating as Nat has said). There is no way you could strike a pane and cause a circular crack some inches from the point of impact with the centre of the circle of glass showing no sign of impact. Whatever caused it they are trying to pin it on the window cleaner to get him to pay for repairs.
Having said that, the sensible thing to have done would have been to inform the customer immediately the damage was noted. That way the customer couldn't have blamed the window cleaner if he hadn't started cleaning that particular window.
As for poking a hole with the pole, that means the pole was reversed and the butt end caused the damage - how likely is that??