This was the direction i was going to suggest...i suspect will be the cheapest/least disruptive way, you could incorporate a box gutter in to the add on part to almost make a feature of it...
half arsed photoshopping below:
View attachment 74441
Thats pretty much spot on and as you only live down the road, fancy the job
Thanks guys for the replies and the efforts with the drawings.
I think i need to speak to a local council inspector to see if I'm aloud to do this modification.