Well, here's my polar - I've got Winter Hill directly to the east, and Parbold Hill to the west, stopping me seeing Dublin!
As to whether you will see aircraft landing at Manchester, that purely depends on whether your aerial will be able to see the tarmac directly. If you can stand on your roof and see the runway, then if you can get an aerial up there, it will be able to as well!
If there are buildings in the way, then forget it. If there are trees in the way, then it will vary according to the season - and wet leaves are much worse than dry leaves, not that the leaves are often dry round these parts! The dent in my polar to the south is due to a big old oak tree.
From here in Chorley I get EGCC traffic down to about 1400 ft.
Rod