This is the Government line, Gordon Brown used this line at Prime Minister's Questions

Let me just read from the mandate agreed at the Council:
'The constitutional concept, which consisted in repealing all existing Treaties and replacing them by a single text called ‘Constitution’, is abandoned.'”. He is absolutely correct, that is what was agreed but that was because it was decided that since the EU Constitution had been rejected by those naughty real people who had been given the chance to vote; then why not drop the idea of replacing all the existing treaties with a shiny new Constitution and instead put the vast majority of the Constitution into another Amending Treaty.
How outlandish, how ridiculous...
Well no, take a look at this,

"The new European Union Treaty has been designed to "keep the advances" of the old constitution "that we would not have dared present directly", a senior Brussels figure has admitted. Hans-Gert Poettering, president of the European Parliament and a close ally of the German Chancellor Angela Merkel, made the admission in a letter to Valéry Giscard d'Estaing, the architect of the discarded EU Constitution.

Full article here