This thread is a good example of how a lack of information from the headline post results in assumptions, and in some cases what appear to be quite wrong assumption being made.
Garry when asking a technical question it is really helpful and important to make sure you include the year, make and model of the appliance you are referring. Not only are there several different makes of water heater available, even what appears to be the same make can have important differences introduced during the products life.
From your description of the "bung at the bottom left" I draw the conclusion you are referring to a Carver Cascade 2 water heater. However there were several different variant of that model, and in particular the drain plug (bottom left) had two distinct designs. The vast majority were of the type and size that Damian has given, but the first tranch used an imperial sized fitting.
In the context of it being a Cascade 2, Mike JE's 1st response makes no sense as there is no red bung on a Cascade.
Forests advice also has no relevance as the bung does not have hexagon to which a spanner can be fitted.
In conclusion, and based on the subsequent posts it does seem to be a Cascade 2 and so an 'O' ring replacement will almost certainly resolve the problem