Agree with Agile my Heatline S12 is more than 10 years old it's drained and filled with an inhibitor every 2 - 3 years and has a magnaclean on it, the diverter valve has just started to leak 40 quid for a universal repair kit.
I found the easiest way to make and fix a plate is: -
Put 2 - 1/2 x 1/2 mild steel right angle along the width of the fireplace chimney so that you effectively make a "L" shape to which your register plate will sit into, if possible these can be drilled but as with most house chimneys the area...