Although unfortunately those heaters seem to remain on all year round even in the summer!
Well, whilst the heater is amazing in winter (like over the last couple of weeks), it is very basic as I believe it basically just works of the coolant loop of the engine! So basically is always on as the engine always requires cooling. I've noticed, during summer, that often when the train is sat in a station idling for a while there heater is not very noticeable but after the train has been in motion (and the engine therefore working hard) for five minutes or so suddenly the trouser leg becomes uncomfortably warm!
Also old northern for whatever reason decided to remove the arm rests from the seats when the sets received their refresh 5 years ago.
They weren't exactly that useful to begin with (they were very low!) and just represented something to be broken if my recollection is correct!