Hi Simon
I've used Doorknockers in the past and have had excellent results. Some members of this forum don't share this view; but I can only comment on my first hand experience ... which has been excellent.
Occasionally, Paul (who runs Doorknockers) is very hard to get hold off, but if you can persevere, it's well worth your efforts.
I'm also thinking of using Gemma from Roundbuilders, who comes across as very professional and reliable - however I haven't actually used her for any canvassing yet, so it's unfair for me to comment on the calibre of her work.
Hope this helps, Rik