(I would have thought an alternative to raising the boiler - in theory at least - would be to lower the straight length's boiler end by an inch or so, and thereby tweaking very slightly the overall 'fall' of the whole length, and then rotating that elbow to suit the angle would also have worked? The terminal end should cope easily with this microscopic tilt as it passes through a rubber diaphragm as it exits allowing such teeny tiny movement. It would probably mean altering the boxing around the pipe to accommodate the drop, tho'. But that's just layman 'theory', as I'm not a plumber, and best to let these guys get on with it...)