Unlikely to have been 13 May - try two weeks later - I travelled on one in the final week - 158756 to Manchester Victoria from London Euston on Monday 22 May 2000 (1150 from Euston) and 158758 back the following day (1526 from Manchester Victoria) - 3 hours and 16 minutes each way.
I think they had been scaled back by then however to just the Rochdale - London - Manchester - London - Rochdale 158 diagram.
Quite likely.
Euston Remodelling went into full swing with the new timetable in summer 2000, and was reduced to 10 (possibly 11) platforms from the summer change date. (It had been reduced to 15 platforms from the Easter). North Western always knew they were going to be kicked out for the summer timetable, and then never came back.