Mate, I dont think you will have to retake the assessments. And if you do, you will only retake the ones that you havent yet done (if any). If you did an interview at assessment stage, that is the MMI and is part of the assessments, it has some similarities to the Driver Manager interview but they are different things; for every application if you get far enough you will have a new DMI with each TOC. The MMI result is held on a national database and is viewable by all TOCs so you dont do that again within a certain time frame (depends on the TOC, usually at least 2 years).
The rest of the assessments (dots, phone book, reaction tests, etcha-sketch, grey square etc) are also held on a national database and you dont do them again if passed unless a long time ago and 'expired'.
The only reason you would have to do any more assessments is if there are some that Northern regularly do for passenger drivers that werent part of your previous ones taken and passed for freight - I dont think there are, but cannot be certain. Good luck tomorrow.