Stove flue

Does anyone have any experience with flue liners. I'm building up my fireplace too accept a inset stove which will have a 125mm liner. My issue is that the front of the flue spigot will be almost plum with the back of the lintel.the lintel sits approx 200mm above the sigot but I also need to get a connector in which is approx 100mm high. In theory all should fit as the liner should run up and just clear the lintel but I just wondered how much flex the liners actually have in them and if I could expect to get any deviation away from the lintel in the space given

why not ask the installer for his requirement as he will be the one signing it off
or iff you are going the building control route you have to involve them before starting