Just guessing, but there is a chance that some sort of sealer may have been used in the fitting of the heater framework to the bulkhead that is now acting as some sort of glue... perhaps if you gently but positively introduce a wall paper scraper into the joint between the two and apply a bit of force something might begin to move..? Wallpaper scrapers can be cut down to a to an inch or so wide if you do not have one that you can fit in..
Good luck Lawson.
