for a combi and removal cylinder and feeder tanks and rearranged piping, with the known names around £3500 all in fixed prices. With some clarification if Cylinder/feed tanks are taken away.
You don't have to have a combi. You can have a heat only boiler (both types are condensing ) then the removal of the cylinder, and its feeder tank with the associated pipe modifications won't be necessary. The same goes for the control wiring, that can stay, perhaps with a new upgraded controller to replace the existing. That would cut down the installation cost; assuming that the existing items are serviceable.