When they put the flyover up in 1970 they forgot it was being built on marshland and Would sink.
The story I got was that it was built using box section steel, which works well in warmer climates, but whoever approved it forgot that Britain is a damp country. The remedial works are to fix problems caused by rust and to strengthen the structure. Where there isn't tarpaulin covering it up, you can see new plates riveted in place at various points.
Agreed that it would have been much better to flatten it and rebuild. It wouldn't even take that long - motorway bridges are replaced in a day or two using pre-built concrete sections which are made elsewhere then trucked in and craned into place.
What gets me is that following the quake in LA and the tsunami in Japan, they had roads and bridges rebuilt and running in a matter of weeks. But here in good old Blighty we fart about for years.