I'm not a heating engineer but here's my suggestion.
Off to the local plumbers merchant and buy several cheap rad keys.
(Lot cheaper than new radiators).
Clean the nips of all grease / oil.
Mix araldite and put a little in each key and glue them onto the nips,
bracing in place overnight with screwed up paper towels or similar.
Then attempt to unscrew.
You will need new nips of course.
hondo, February 2009