Hmmm, yes, 'tis a 'mare of a building, isn't it? You can see why I asked for advice now, can't you?
You've all done very well. (Thank you Mr. Grace. That'll be completely lost on many). I did my efforts using CS2 lens correction tool, using a combination of correcting the perspective and barrel distortion. As Don says, the more the top is corrected, the worse the bottom gets.
I woke up this morning (at about 5:00 am!) with the idea of correcting the perspective/distortion on each of the two parts of the joiner
before stitching them together. There is actually very little perspective distortion on the bottom picture, as you'd expect, so applying the major correction to the top before stitching should stop the bottom going so squiffy. I'll let you know how I get on. Probably about midnight!
Duncan.
P.S. Rob, I had noticed your "birthday" too!