The reason for this is the material from which I made the jig. To save money I made the jig out of wood, however it has been a month or two since I made the jig and mounted the aluminium fixtures on to it - in the correct location I may add.
The issue is that, having re-measured the jig prior to continuing with the frame, it would appear that the jig has swelled ever so slightly. This in turn put all of the aluminium fixtures out of position!
Having re-located the fixtures to the correct position I then continued with brazing up the BB shell - see pic below:
Next up I added the top tube and down tube to the jig, tacked it all together and fully brazed the seat tube to the top tube.
Tomorrow I will be finishing off the main triangle brazes and shaping the rear triangle tubes ready for their joining to the main triangle.