First get the name of a contact and phone number and email - by walking in and offering your services of course.
Then if they are at a different office or it's too big an outfit to have a quick chat there and then send this by email and post:-
Mrs Felicity Mange
Facilities Manager
Meglacorp PLC
Gasworks Trad Est
Chepstow
Dear Mrs Mange,
We are the premier window cleaners in/on (trad estate/area where you want to get the work) and respectfully offer our services to your organisation.
We currently clean XYZ Ltd, ABC PLC and Minicorp on your estate (in your area) having cleaned the latter for XX years and we specialise in reliable, regular cleaning at affordable prices. Obviously we can tailor our services to your requirements.
I will phone within the next few days to discuss what we can offer or arrange an appointment to provide a quotation.
Kind regards,
WC