thanks for those very helpful descriptions.
I've replaced the ball float as this was full of water - old one probably worth a bit in scrap!
I managed to get the old washer out - it was totally disintegrating, but found it was not a standard size. it sits in a two piece metal cylinder with a collar, which i unscrewed to access the washer. I cut a mm or so off two new ones to fit into the space of the old one - it seemed the old one was thicker. After i screwed the washer piston back together again, the new washer curves outwards slightly as it is gripped by the metal collar - I hope that doesn't mean i overtightened this.
I guess to access the nozzle will require me to remove the valve and access it from the back of the valve? I'm torn between attempting this as a learning project, and just saving my time and getting someone in to rework the connections at the back.
Oh, and the new standard sized type 2 valve unfortunately has a thread connector of a smaller diameter than the existing one. I realise this means I got the wrong thing, but was there an older standard for this?