Can't say about MAN-SHF for definite. Whilst I've heard that overcrowding is bad on this part of the route I've never experienced it myself, but seldom use this part of the service. Certainly south of Sheffield towards Nottingham, the part I use most often it can be rather busy at times but I've always been able to get a seat. It's two 158 sets coupled together north of Nottingham but one detaches at Nottingham, from where the service is less busy. All in all I've not experienced overcrowding on this service but have never travelled on it out of Manchester at the time you suggest.
Has it always been the case that two DMUs were coupled together north of Nottingham I wonder? If there was a period when it wasn't I suspect overcrowding would be very severe indeed.