Anyone know where to get collets suitable for this type of valve? Or is it a different arrangement?

ESM only seem to have the ones to suit the "long grooved" ones....
If the engine is free to turn over, remove the spark plug from the selected cylinder, check that the piston is on the way up, stuff a bunch of soft rope, into the cylinder, through the spark plug hole, leaving the end sticking out for later removal, turn the engine over, bringing the piston up to pack the rope between the piston and the head. The valves are now held up in place so you can compress the spring and remove the keepers. Simple.Any idea how to get them off without removing the head?