Congratulations TSB Another waste of Money
Firstly, WTF does a law firm know about IT systems, Banking platforms, Migrations methodologies, strategies and testing procedures
Secondly, as already mentioned, the number of defects is irrelevant, it is the number of showstoppers. I don't give a damn if there are 5,000 in the queue if they are all category 5 such as "Snr Mgr can't change colour of menu bar". We've all had ones like these. The question that needs to be answered is "How many showstoppers did the decisions makers know about & downgrade so they could meet the go live date and so as to not incur financial penalties?"
Thirdly with statements like this "we specifically asked for a fully independent and thorough inquiry. Although the report doesn't paint the full picture of migration"
If the report doesn't paint a full picture of the migration then "IT"S NOT A FUCKING THOROUGH INQUIRY"
I'm surprised you overpaid wankers can tie your showlaces in the morning.
Finally, If the IT contractors had multiple successes in data migrations before, the argument can simply be made the only variable were the the overpaid tosspots , aka PHB, at TSB.