One possibility could be to put the boiler inside the existing footprint of the house, run the flue up through the ceiling, through a hole in the floorboards in the corner of the spare bedroom above, and then turn it 90 degrees to come out the wall horizontally, above the pitch of the extension...
Thanks Muggles for your helpful response. Unfortunately I probably should have been clearer that the ground floor roof is a single slope, as it's a one storey extension at the back of the house. So the ridge tile won't work (great idea though!), not the chimney. Any other ideas?!
I'm currently renovating our house and as part of that we are installing a new boiler as the old one illegally vented into next door's garden (trespass).
The plan is to terminate the flue going vertically up out of the 1st floor pitched roof. However, because there are large velux windows...