Install a new "o" ring (available from e.g. eBay).

I added liquid gasket to make sure the pitting was not going to cause me problems. In my case I used Loctite SI 5980 which is avaialble from motoring shops. See the key photo.

Present the top to the bottom of the heater, there are keys so make sure it is the right way around. Gently push together, and adding the coffee press body to the bottom of the heater, put the four bolts back into the heater.

Tighten them up on the opposite corners. e.g. there are four bolts, say one is North, one South, East and West. Tighten the North until it grips. Then tighten South until it grips, followed by East and West. Then Tighten them up a little more. You should see a little liquid gasket come out of the joint all the way around. (photo 2)