I had a steel bike in the late 80's - columbus SLX tubing IIRC (lovely bike, had the name of a shop in bolton on it, one in the centre, not Geoff Smith-can't remember the name).
It started to flex around the bb after a fair few years use (was a second hand bike). IIRC it was Geoff Smith that told me this was an issue with the [steel] frames (at the time he put on one very first sealed Mavic bb's on it). Is this correct that they start to flex after a while?
Man, that was a lovely bike - very similar colours and finish to yours Fossy and probably similar parts too IIRC (Dura Ace, 600, Open 4 CD's). Wish I still had it :-(