The decision yesterday by the House of Commons to choose John Bercow as its new Speaker will either be a triumph, or it will be a disaster. It is difficult to say which. But given the severity of the Parliamentary crisis and the character of Mr Bercow, a quiet mediocrity seems unlikely.

Mr Bercow is young, eloquent, liberal and an avowed moderniser. These are all attractive characteristics. He is obviously superior to his predecessor. He may be able to present himself to the public as the clean-break candidate. There is every chance that Mr Speaker Bercow might be remembered as a great holder of the office.

Yet the possibility that it may go dreadfully wrong is also present. Disaster may take two forms. Mr Bercow may overdo it. Or he may underdo it. He could even do both at once.

Listening to the new Speaker address the House before the first ballot it was impossible to remain unimpressed. He speaks fluently and he speaks the language of reform. There is, however, always a danger with Mr Bercow that no one will be more impressed with one of his orations than he himself. The House of Commons does not need a Speaker fonder of hearing his own voice than hearing the voice of members.

The other way in which Mr Bercow could overdo it is to persist in the poor relations he has with members of his own party. Tactfully, but urgently, Mr Bercow now needs to work to ensure that he acts as a Speaker for all the House rather than a practical joke played on David Cameron by Labour MPs.

Mr Bercow has the personal qualities to avoid these pitfalls, if he recognises them as such. A bigger challenge may be to be a genuine reformer. The new Speaker talks of change, but is equally fond of talking of history. His speeches are full of parliamentary tradition and he is painfully aware of his many great predecessors. The danger is that he may be the prisoner of tradition, of precedent and of a House standing on its dignity.

To avoid this, first he must understand that politics needs an advocate and a leader. Speaker Michael Martin was a shop steward rather than a statesman. Mr Bercow should be a determined moderniser, impatient with traditions that get between Parliament and the voter. He should insist on greater civility in the House. He should always ask himself how parliamentary practice looks to those who do not sit in the House.

This, obviously, also means leading his colleagues towards change on pay and allowances. It also means thorough reform of the bodies that let the whole fiasco happen in the first place. The administration of the Commons needs thorough change. The demeanour of staff at the Palace of Westminster must change too. Visitors should be seen as the owners of Parliament rather than unwanted strangers.

Second, the Speaker needs to deliver on the promise made by all the candidates to hand back power to the legislature from the executive. Mr Bercow should not allow tradition and deference to prevent him from getting his way. He has to be willing to be bold.

This leads to a third requirement. If the Speaker is going to deal with controversial issues he needs to be removable. Among the first things Mr Bercow should do is to put in place a means of deselection. This would allow him to fight tough political battles with the executive on a more equal footing — it would take more than a few murmurs of discontent to destabilise him.

There are two sides to John Bercow. The passionate, articulate moderniser. And the traditionalist who wants to wear the Speaker’s garb. From tomorrow he should announce that the garb is for fancy dress occasions. He will be the plain-clothes, plain-speaking, plain-reforming Mr Speaker.