on Monday 2nd April for the 6:22 Newcastle to Glasgow?
I'm aware this is usually a peak time train but was not aware if the bank holiday changes anything
There is no such thing as a peak time train. Whether or not a train is allowed on an Off-Peak ticket depends on the restriction code of the ticket in question.
That being said, as I understand it all, or at least most, Off-Peak restrictions are lifted on bank holidays, at least in the electronic data version that journey planners use. This is despite the fact that many of them do not include such an exception in their text.
If you can get the Off-Peak ticket you have (or intend to get) - making sure that is the right Off-Peak ticket, as there are several different ones on most routes - to come up on
www.nationalrail.co.uk when searching for the train you want to take, then it'll be valid.