It is a pointless act of defiance. It undermines the importance and heritage of the occasion and makes the sartorially challenged Mr Brown look like an inverted snob.
This week the Corporation elders will decide how to respond. My guess is that they will opt for appeasement. Invited guests to the Mansion House dinner will probably be allowed to decide their own dress code. This would be a mistake. The Corporation has some bigger and more important issues to resolve with the Government and to cave in on the question of evening wear will send the wrong signals.
Let Mr Brown wear what he wants. If he wants to be a monkey rather than in a monkey suit that is up to him. But he should not be allowed to undermine the linkage between tradition and stability which is an integral part of the City's reputation.