Thanks for your help griff...... All sorted now..... No more knocking!!!
Looked at your pics which were a huge help ad did as you did. My captive nut was fine and the bolt came out easily. Once I had cut my access hole I could see that everything in there looked in really good condition. Took out the bolt cleaned it up a little and put it back in. Even with the bolt as tight as I could get it there was still a lot of movement in the subframe. I jacked up the body and loosened the other subframe bolts to lower the subframe a little. It them became obvious that in my case the cracks were in the underside part of the chassis rail not the inner side where the captive nut is. Basically I was tightening the subframe against the face of the bolt tube which because if corrosion and cracks ws now proud of the chassis rail..... If that makes sense.
Anyway..... Cut mymyself a suitably sized piece of 3mm plate and made a repair piece. Slotted it in, tightened up the subframe bolts and hey presto.... No more movement or knocking. Then welded the plate in place. Thinking about it, just wondering if it is corrosion on the bottom of the rail allows the movement of the subframe which then causes the cracks in the top of the rail??? Just a thought
Thanks again griff
Chris