Approach it by first thanking the person that gave you the information, then when you email the manager introduce yourself and the business, then refer to the person that directed you to them.
Eg Mr/Mrs manager/owner, my name is blarg of blargs window cleaning, I was told to contact you by (name of first contact and their location) to see if you had a current window cleaner or if you were happy with your current one?
And then your contact information, now remember don't forget to follow up!
For example: Me and the wife Just bought a car yesterday; we had to wait for it to be fixed first and my wife hadn't received a phone call on Tuesday when the guy said he'd call. Well, she called him back and apparently he had lost her number
This isn't the only time: went to apply for an apartment, the guy that did my application was either fired/quit and when we called to get the move in date they didn't even know the guy had left pending applications