Give the OP says break. She asked a simple question, and the job itself is not difficult. The biggest problem will.be the lintel restriction, as pointed out. The fact that the original install is rough is besides the point.
As for Tony knowing the OP had a budget of £50, I assume he has polished his balls again
Op: Your probably cant go much higher and if the flue gets damaged you will have problems.
So would would you move the boiler and refit using the existing methods as seen?