Yeah, as above re tradesmen entrance but often only works 7 am to 9 am - so go with cards and put them through the letter boxes without knocking or you'll get chucked out.
Plan B
Press any bell say "I've got some correspondence for flat (No. whatever - not the one you're ringing) can you let me in so I can put it through the door?" 9 out of 10 they'll let you in. Put one through every door. When you've got one you're in to canvass properly.
Plan C
Wait for the Postie, give him a fiver/tenner for his trouble and ask him to drop an envelope for each flat with a short letter in it about your services.