Can I challenge the amount I have to pay under a "no win, no fee" agreement?

The Senior Costs Judge, Master Gordon-Saker, says yes, although it will be dependent upon the particular circumstances. 

In this case the hourly rates were too high - far higher than would have been allowed against the other side in the event of a success, and the success fee was not based on any assessment of the risk of not winning the case.

Although not a binding decision, this will be highly persuasive in cases where the solicitors argue that there is no entitlement to go behind the terms of a CFA on grounds that it is contract.

As with all retainers between solicitor and client, the terms must be fair and must be properly understood by the client.

Full judgment below, followed by full article from the Law Socity Gazette.