To answer my own question - after struggling with a number of different valve spring compressors then talking to Dave Owen and borrowing his modified compressor (I have modified my own now too) I have managed to fit the collets with the aid of a large socket over the valve head (difficulty compounded by it being a blind barrel). So it is possible without losing an eye or finger...
Swindon Branch Secretary
Ariels - 1913 TT Model, 20 Roadster, 28 Model D, 30 Model B, 38 VH, 52 VH
The Offroad Fleet - Fantic 200, Bultaco Pursang, Beta Rev 3