I was looking at the gasket today and I wasn't happy with the way it hadn't sealed and it was quite clear to see that it was not making a very good seal. Now, when I took it apart I remembered the screws were quite tight but in very good condition so the only thing I could think may have caused a problem was that somebody had replaced the screws with slightly longer ones. Basically, the plate is three washers thick so I decided to cut my new screws two washers thick.
As you can see from the picture, had the old gasket in place and with the screw fully screwed home I could squeeze two washers under the head.
This picture shows the original screw, I can quite comfortably slide three washers under: they are only stepped for the purpose of the photograph.
This will give you some idea of the difference in length. I believe this would make a large contribution for why the heater was not working.