I was being dense. There are three threaded mounting holes in the rear pillar when you remove the soft top mounting bracket. The third (bottom one) had never been used and was still covered in headlining and had been filled with glue from the factory (one side not threaded in true BL style….) The catches on the hard top fix with one bolt on each side to this bottom threaded hole. My car is off having its IRS rebuilt but when it is back I will post a series of photos to show the procedure.
If anyone knows what thread size these holes are (the same as the pan head soft top machine screws on the XJSC) it will make my life easier when I get my car back