Try Citibase on Princess Street, they are usually the cheapest in the City.
Failing that, Yourspace is also good, as they do deals which includes free phone calls.
You could try Piccadilly House just down from Piccadilly Train station.
If you are moving into Manchester for the first time you could try contacting MIDAS who manage inward investment in Manchester, they will be happy to escort you round and show you some interesting places.