Manchester has a spotters viewing park which you have to pay to get in, not too sure how much.
Concorde is parked up there, also a BAE 146, an old military jet and a Cockpit from a Monarch DC10. There is a spotters shop with books, models etc etc and refreshment stand.
Best days in the summer season would be weekends due to the amount of rotations that airlines have their aircraft working. Most fly out in the mornings then back midday, turnaround in about an hour then head off again.
Hope that helps.