I've worked on virtually all the loco's mentioned above and I can safely say that they can all be equally as cold as one another. I remember climbing into the cab of a DMU with an old Darlington driver and the heat inside the cab was tremendous, so I went to turn one of the heaters off. I didn't get a chance to because he bellowed at me "DON'T TURN THE HEATER OFF COS I MIGHT NOT BE ABLE TO TURN IT ON AGAIN!!! IF YOUR TOO HOT, OPEN THE WINDOW!!!!"
The thing I found with loco and DMU heaters was that they were fine as long as they worked and there were no draughts. It was the draughts that made the cabs so cold. On one occasion when I was taking a DMU off Thornaby shed, I went to the stores and got a roll of masking tape and spent the next quarter of an hour sealing as many draughts as I could find. By the time I'd finished, there was more masking tape to see than cab fittings, and I'd used virtually the whole roll, but the cab was warm as toast.
On another occasion, I was on a Class 37, and although the heater next to me was too hot to touch, move more than an inch away from it and it was freezing. I hunted around the cab and found a draught coming into it from somewhere near the second mans footwarmer. There was a hole in the floor there. I didn't have any masking tape so I got a newspaper and crumpled up a sheet from it and stuffed it into the hole. That disappeared, so I stuffed another crumpled up page into it, and another, and another, and another, until the newspaper was gone, and yet there was still a draught. So then I used my weekly notices ( I always knew I'd find a proper use for them one day
), and proceeded to stuff pages from them into the hole. Finally I got to the point when I couldn't stuff anymore paper into the hole, but by this time the draught had stopped, and again, within minutes, the cab was as warm as toast.