Thanks for the other suggestions, the penetrating oil did the trick and with a little gentle encouragement the ring freed up.
Took the rings off and cleaned them up and the piston.
Gave the bore a light hone and painted cylinder and head.
Looks ok to me with the Hycote Extreme Heat Aluminium paint, I'll let everyone know what it's like long term when I get the bike finished.
Fitted back on the bike tonight in a rather chilly garage.

That paint looks nice! Good job on being patient and not breaking the ring or something.

Yes the high temp paint looks good! I've had good luck using high temp clear on nicely cleaned heads. BTW did you check the ring gap in the cylinder in a few spots? Hate to see good labor go to waste if rings/cylinder were worn beyond spec. Check for ring wear down low in the cylinder beyond the travel. Check for combined wear any where in the travel.

Got the wiring done, thanks to DEET for supplying some more sleeving and forwarding a cable set from KDI.
Checked that I still have a spark and I do
Exhaust ring cleaned up after a long soak.
Rectifier and horn received a lick of paint, more screws nickel plated.
Oil tank and clocks on.
Popped the tank and seat on so I could see how the bike looked and I have to say I'm really liking the colour, I don't think the photos do it justice.
Anyway that's it for now, she's coming along nicely.

where did you get the tank badges from? i am looking for a set for my JT1

I got them off ebay, I've just had a look and the seller is out of stock.
Hope you manage to find some, Pete

Wow what a cool color scheme. I dont think ive ever seen anything quite like it, Looks killer tho.

The quality of this restoration is very impressive. Great work Pete, that thing is as clean as they get.

Looking bonny now Pete.
It's good when you get to that point.
Paint looks pretty good too Pete
Dent tool worked too on my CT3 . The compression on the CT3 is really good but the kick mechanism is driving me a bit daft .
Sometimes it's picking up where it should other times i have to put it to the bottom of it's stroke .
It's bloody freezing here today, how's it down your neck of the woods?

All good here thanks Dave, how are you doing?
Aluminium plate arrived today and I was gonna start on my leak down tester, but it was such a nice afternoon I decided to take the Greenie out for a ride instead.
44 miles was warming for the soul but cold on my fingers.
I get the impression your CT3 has not been the smoothest resto.