I've just fitted a new valve to my boiler. I used a normal length parallel nipple, one end in the valve the other locked off with a 22mm compression to 1 inch BSP adapter. That way it is held firm and I can connect the 22mm hop stopper easily.
I had to go 1 inch as the original valves on my boilers were really crappy 1 inch jobs.
This is what it looks like fitted. I don't have any photos of the individual parts but I used...
1 inch BSP Ball Valve
1 inch BSP Parallel nipple
22mm (compression) to 1 inch BSPF adapter
1 inch to 1/2 inch BSP hexagon reducing bush
1/2 inch BSPM to 1/2 inch hose barb
/Phil.