Hi Pete,
Do a search for Alpha Bearings in Dudley, West Midlands. For many years they were the one company that offered outstanding service to guys like us with engine problems. They manufactured and repaired roller big end cranks and also offered a huge range of motorcycle valves.
Sadly the original owner is no more, but the business is now under the ownership of Autocycle, another long standing local name. The valve catalogue is still on line, but whether they are actually trading at present I do not know.
Should point you in the right direction.
Take care matching the collets to the valves, as there are variations in the size and profile of the valve stem where the collet fits.
Swarfy.