dont use a haksaw whatever you do ! it will distort the cut end of the pipe , use a pipe cutter - quite cheap from B&Q , then using pre soldered straight couplers lengthen the pipe (use a gas gun) then cut the pipe to length with the pipe cutter cut the pipe to fit into the rad valve.
Unless you have done this type of thing b4 , maybe best left to a plumber !
big shaun, December 2008