Smurf's comments and SeanK's reply got me thinking.
That's a whole new meaning to builders cleans 
I take it the roofer chap was not insured properly then hence why has not provided his insurance details so they are trying to put the blame on you now. I'm thinking a Solar Panel Cleaning Waiver springs to mind on that one Steve and I'm sure you got them to sign one.There's more to this story than meets the eye, Smurf a cleaning waiver has about as much legal value as a car parking waiver
in other words none, they're only there to deter a customer claim.There is an obvious risk in removing cement, limescale, baked on bird droppings, paint etc etc of damaging the property. Where we regard this as a 'high' risk - removing cement or paint by using a scraper we usually ask the customer to sign a Damage Waiver. If SeanK is right, this doesn't protect us from being sued if consequently there is damage to the property.
To try to clarify the situation I phoned the FSB Legal Advice service. The reply I got was that if the Waiver states clearly the risks involved in us cleaning off the foreign matter and if the customer understands and accepts that they will be forfeiting any claim for damages resulting from us undertaking the work at their request then we would not be liable. The only way we could incur liability would be if the damage were due to our negligence, not the circumstances specified in the Waiver.
The solicitor's advice was that if we were in any doubt as to the wording of the Waiver then we should have a solicitor look at it prior to getting the customer to sign.
A properly worded Waiver is a valid and necessary means of protecting yourself from a possibly very expensive claim for damage to someones' property.