In 99.9% of tenancy agreements I have seen, broken glass is the responsibility of the tenant regardless of how the damage occurred.
Just one more point on agents however, the government will not legislate to regulate the industry. This means that anybody, and I mean anybody can set up an agency and suddenly find they have lots of responsibilities to landlords and tenants, and also hold hundreds of thousands, if not millions of pounds of other people's money, with no accountability to any one.
There is legislation to protect that money, but it isn't policed.
So if you are renting or a landlord, make sure you use an agent who is licensed by ARLA, and is a member of the Property Ombudsman, if it goes wrong these two will sort the issue out.
"By skill not force."