Could a 'sunset clause' set tax vote deadline?

Sources have been suggesting that a draft Wales Bill to usher in the tax and borrowing powers announced by the Prime Minister last week could contain a 'sunset clause' which would set a deadline for holding a referendum on transferring the power to vary income tax.

The Conservatives, Plaid Cymru and the Liberal Democrats are all keen to see such a referendum held within the next few years, but senior Labour figures such as the First Minister, Finance Minister and Shadow Welsh Secretary have refused to back a vote without the promise of fair funding.

The Silk Commission report said 2017 was the 'earliest' date for holding a referendum but it's now being touted as a likely one. A sunset clause would mean that Assembly politicians would have to agree to trigger a vote by whatever deadline was set or lose the right to do so altogether.