Thanks for that. Sorry not to reply earlier, was away in France for a couple of weeks. It was the bottom one that i needed, i managed to get it off by crushing the rusty nut and tapping out the bolt. The damper has become pretty out of shape and like you said, the part is not in stock. I did wonder if it was the same as the bottom vent for the 47cm compact kettle sold in the UK.
I ended up taking my Go Anywhere, after cutting the grate in half to increase the storage capacity for travelling. It worked great but i do miss the capacity of the Jumbo Joe for being able to quickly feed kids a pile of sausages.
Having looked at my JJ, the bowl is pretty rusted where the handles go through the holes so i may have to bite the bullet and order a new one from the US. Just checked price and it would be £120 delivered to the UK, blimey!