Loose rivnut in frame?

I have a loose rivnut (also called a nutsert) in the frame of my ZX7R project.... it's one of the two fuel tank forward mounting bolt rivnuts & is so loose that when I try & screw in the bolt it starts rotating the rivnut after only a thread or two.
I don't know how they installed (probably installed before the frame is welded up!) & am hoping there may be a special tool which could be used to clinch it tight in the frame again? Obviously there is no access to the underside of the rivnut.
Has anybody had any experience fixing these things?
Jason
I don't know how they installed (probably installed before the frame is welded up!) & am hoping there may be a special tool which could be used to clinch it tight in the frame again? Obviously there is no access to the underside of the rivnut.

Has anybody had any experience fixing these things?
Jason