I've just purchased a towel rail radiator to replace an old bathroom radiator. The problem i have is the towel radiator is only 35cm wide whereas the old radiator was 45cm. Also the valves screw in vertically (from the bottom) and the current valves screw in horizontally. The pipes to all the radiators in the house are 10mm and are behind the wall (plasterboard) rather than under the floor boards.

Is there an easy way (suitable for a novice plumber) to replace the valves and reduce the width between the valves from 45cm to 35cm without draining and disabling the central heating tank in the loft? Or could someone point me in the right direction to an idiots guide on this?