jigman, If you can't find an exact match regarding the width of the radiator and you don't want to alter the pipework, your new radiator would need to be smaller in width. To compensate for the difference, you can aquire radiator valve extensions ( chromium plated ),from Wickes etc.
Also, in order to compensate for the heat output difference because of reduction in width of radiator, you may wish to aquire a taller radiator, or select one with fins or possibly a double one.
Hope this helps.