After doing some further on bench testing of problem pieces upon further careful examination it turns out the main body of the carb is warped at the top where the air horn attaches. Quite badly too. By putting a straight edge over it was all to clear that the point where the vacuum ports for the power valve mate were very low. I now believe that this produced a vacuum leak and non functioning power valve.

All of those little "ears" that are cast into the main body that the attaching screws are inserted into have pulled upwards after many years of dismantling and re-assembly.

I have now run it over a surface place and trued it up again so will re-assemble and do some testing again later today to see how it goes.