Right BB joint time. The Mite has been checked:
The preparation it sand the outside and inside of the frame. The wipe down with Propanol. The bottom bracket needs a 15mm vent hole. then its time to set it up on the bench to get it square slide it all half off the bench so there is room the weld and tack it and check it. Then get the whole joint warm then wet it in outside to inside, then check it.
All square. The seat tube needs re-enforcing 0.8mm tube silver soldered to the seat tube.So the total thickness is 1.8mm which should do but not be to much extra weight.it can be cut to lenght after.