Just a thought, why on earth don't people canvass for themselves? If I was the potential customer I think I'd rather be in contact with the guy wanting the work from the outset. If you want blocks of flats then pop in yourself and find out who's in charge of the maintenance then drop them a line or phone.
£400 to get someone to pick up the phone is ludicrous to me. If you've got the gaps to take on the work then you've certainly got the time to press a few doorbells or dial a number.