Re: Bad before it started.
I'm hearing rumours of a a similar story to what someone suggested down below.
They have apparently tried to do a simultaneous migration and upgrade. Rumour is the migration went okay (which is why Sabadell are publicly praising it) but it's the upgrade that's gone Pete Tong. It would apparently be quicker to forge ahead and fix it rather than rolling back.
Given that people are seeing mortgage balances that are millions of dollars in arrears, this suggests default configuration and one badly mangled dataset. TSB doesn't handle dollars and nobody sane has a mortgage that large, let alone that far in arrears. That is spectacularly broken.