After a brief wait for some more 50x25 box section (due to the fact that I cut up the wrong bit...) the base is nearing completion!  The two rectangles have been tacked together, and the two long bits (technical term!) will be bolted on.  This mean that I'll be able to disassemble it and hopefully managed to get it through the door.  The UJ then bolts onto the plate, and it should start to look somewhere near its final form.