I have replaced a section of cast iron downpipe (68mm) with plastic and everything fits well. The problem is as the bottom section is still the old cast iron, the pipe is just a few mm away from the wall. The only brackets I can find for the plastic section put it at about 3 cm away from the wall, what can I do?

Not sure whether I understand what you are asking... Anyway, Plastic pipe uses different brackets ( vertical and horizontal ) than those used for Cast Iron. If Plastic Pipe is 3cm away from the wall than I would use 7/8" Unistrut channel with appropriate Unistrut Clamp for Plastic Pipe. Can be used vertically or horizontally. You can purchase this type of materials in stores that cater to plumbing contractors. ( see the attachment ). Hope that helps. Back to you. Milo