I have an Alpha CB 24 Combi Boiler. The flue emerges through the top of the boiler and then bends immediataly at 90 degrees to go horizontally through the exterior wall to the left of the boiler. The flue then emerges horizontally from the wall 210 cm from the ground (base of flue), protruding by 8 inches, with a cast aluminium slotted cone for dispersal of gasses, adding a further 4 inches. Because of a plastic rainwater gutter above the flue, a sheet of aluminium has been placed 6 inches above the flue as a thermal guard between the flue and the guttering.
I wish to make a lean to garage with a polycarbonate roof on this side of the house and I was wondering if I could add another 90 degree bend to the flue, taking it vertical and above the guttering? Would I require an engineer to make the alterations and or to test for leaks, or would it be possible to diy buying in the appropriate flue pipes?

No, get a professional in to look at it.

You cannot do that anyway!

Elbows can only be used internally.

A special vertical flue is needed and has to rise up through the building.

Tony