I recall someone suggesting this already, now who was it?...the pin gets stuck inside over the summer when not in use which is exactly what happened to the radiator in the main bedroom.
I'll have a go at tapping the pin to see if I can release it as a few others have suggested otherwise I'll just have to replace the whole valve. Argh scary!
Pull the pin up with a pair of pliers. But why not just bite the bullet and replace valves. It'll be worth it, and it isn't difficult. Just take your time.
(Edit: Acknowledge that Bengasman's coin suggestion is indeed safer)