HELP! - Downpipe connection query

[samsocket]

I'm replacing my guttering & downpipe. However, about 4ft from the floor I want to tap my central heating condense pipe into the downpipe. The condense pipe is hard plastic and much smaller bore (about hosepipe size bore).
There does not appear to be such a connector available.
A water butt diverter type thing would be ideal but I don't want to divert water out, I want to feed water in!
Any ideas please would much appreciated.

 

[datarebal]

what happens if the downpipe blocks at the bottom? i think it could back up into the boiler/house... but i'm not a plumber/heating engineer