Where Is It Then?

This morning when being interviewed by Andrew Marr, David Cameron said the following (here at 36mins in):

"We have put in place, you know, a referendum lock"

Put in place? Well where is it then? I can't find it. There's no draft, no publication and nor does it appear under the current list of Government Bills (They've manged to get to the 2nd readings stage on the Fixed Parliaments and AV referendum bills though).

The last reference to a referendum lock was a statement to the House which made this promise, with no timescale:

we will introduce legislation to ensure that any subsequent future treaty that proposes to transfer such competence or powers would be subject to a referendum of the British people before it could be ratified by this country.

Meanwhile the steady erosion of our sovereignty continues.