There's a section in the latest DVD, Year of the Armoured Car where John Belfield reveals how he frees stuck valves (available in NTSC and PAL)
Basically he uses a bent screwdriver to gently ease the valve down after applying a penetrating lubricant....
The caution is to be very careful as it is of course possible to bend the valve stem in which case you'll have to remove the heads and valley cover to remove the offending valve and guide assembly.