Typical, they spent time fixing it only for it to fail a week later!
I suspect that the problem may have been caused by rapid expansion of one or more of the bridge components due to the sudden onset of very hot weather.
Is the Brigg line, with its multiple signal boxes, even ‘open’ on a Sunday?
That's probably right. Either the bridge won't quite reach the position where the locks can engage, or they do engage but not far enough for the limit switches that detect that the bridge is in the right place and locked to operate. Having commissioned a few swing and lifting bridges in my time, that's the kind of problem that happens. (Solved by the foreman of the Scherzer lift bridge at Waterford many years ago by getting a few lads from the pub to jump up and down on the end - not really on for a railway swing bridge, though)