What could possibly go wrong?
Some of the many, many, many problems with the project:
1. Govt was taking responsibility for integrating the systems and suppliers together as the big consultancies were ripping them off doing this type of work. The govt clearly has lots of experience in successfully delivering large scale IT projects pulling lots of complex IT together from lots of suppliers. So no one SI to rule them all, lots of smaller chunks of work from companies who are far more experienced at very, very clearly defining what their responsibilities were and making dammed sure that the govt or any other SI couldn't shaft them.
2. Requirements that were pulled out of someones arse and constantly changed as govt realised they got it wrong. Well since the govt is the prime contractor they bear the costs and the responsibility. Welcome to the big boys and what risk management is all about :)
3. Cabinet Office sticking their nose in and dictating Agile development. Then Cabinet Office legging it as fast as they could when that didn't work and leaving the team on the ground to take the blame.
4. Unclear leadership at the top. IDS is not, lets be polite, the sharpest IT mind in the govt. Shouting and ranting at civil servants may look good in the press but doesn't really endear him to them and get them to pull the stops out to deliver.
5. A succession of govt delivery leads who promise lots and then realise the utter dungheap of a project they are trying to deliver.
Mind you, you could take this list and apply it to most govt projects now and in the future.
Hope this helps.