OK, did the job - for anyone that needs to do the same here is what we did and (more or less) worked. Removed the wooden bracing bar in the stbd aft cabin, released screws (both port & stbd) from cross wooden bulkhead that is aft of engine and infront of water heater and removed (tricky). Removed side wooden panel between water tank and water heater. Disconnected water heater electrics and then hoses (including heat exchange hoses from engine. (lots of water so have plugs handy). Unscrewed water heater. Then you have to juggle the unit around - we had to leverage the wooden panels that seperate the aft berths (flex them until you can squeeze the tank out). Replacement is basically the reverse of this. Word of caution - the spigot that provides water to the shaft seal is very fragile and can easily be broken off - so beware. Good luck to anyone else doing this.
