Finally, problem solved.
That boiling spitting hissing sound that was coming from the back of the engine (where it meets the bulkhead) - while there was no water squirting out nor any steam, but it was tiny amounts of water just boiling away. So yes, there was a tiny leak in the head gasket.
There was only one thing for it - a bottle of Wondarweld. A friend told me to dump it into the tube close to the engine block so it circulates quickly into there so I took the hose off the Rad on the right hand side (down where the date and time are in my engine photograph). The Holts Wondarweld did the trick and no more spitting or hissing and no more water loss. Obviously, a hell of a lot better than needing to get a head re-skimmed.