Dave, seriously now. You need to be rational about this, you won't solve it, well, not without removing that rad. I'm 99% sure of that. It's one of those things where you'll end up going round and round in circles trying to find a solution and there isn't one. I've been there myself and just wished I'd done it the obvious way.

I'm afraid to say, the only way you'll effectively deaden the sound is to take the rad off, batten the wall, place some 50mm (approx) high density acoustic insulation in there and board over it.

I know you'll think I'm a cunt for saying it, but I really think I'm right here. Just bite the proverbial bullet.

Edit - Just read your post above, same effect. That'll work.