Modern etiquette puzzler

What is the correct form of address to use when writing to a company? "Dear Sirs" may have been acceptable back in the days of Reginald Perrin, but nowadays feels uncomfortable at best. In this case, I know that almost all the staff are women so it feels even more wrong.

N.B. as the company in question is a letting agency, I have considered "Dear Parasitical Bastards," but felt it (a) would not meet with Miss Manners' approval and (b) could result in the speedy termination of my tenancy and enforced relocation to a cardboard box on the hard shoulder of the M11.