I lived in Manchester a coupe of years ago and when I even did go spotting, I went to the top floor of the multistory car par at terminal 1. The easiest way I got to it was to get the train to the airport and take the passenger walkway to terminal 1, once I reached the end of the walkway and got to the terminal I took the elevator to the top floor of the car park.
You can catch all the movements at terminal 2 and most of the runway action, I cant remember if you can catch much action at terminal 1 for some reason, but it is a place I found easy to get to.