Part for Glowworm Energysaver boiler?

I have a Glow worm Energysaver condensing boiler, it appears that the lower heat exchange requires replacing, however the screws at the back appear to be rusted in?? I have been warned that it is possible the screw heads will break thus requiring the whole cabinet to be replaced in order to change out the heat exchanger??
Where can I find a replacement cabinet for this boiler??