Dont deny it, dont talk to the customer. Just sort it out. You cant prove it wasn't your guy and the custard cant prove it was. But you guy was probably the last person on the glass.
It'll be £60.00. It's pocket money. In the grand scheme of things what's 60 squid these day to us lot?
Do the right thing, be co-operative, don't resist what everyone tells you, just go with the flow. Be nice about it. Repair it, sort it, shake the customers hand and buy them a bunch of flowers for their inconvenience.