The Government doc The Future Development of Air Transport on the UK : North of England (2002) had this re population catchment areas.
1 hour road:
LPL 5-7 million
MAN 5-7 million
2 hour road:
LPL 13-15 million
MAN 15-17 million
1 hour rail:
LPL 0.4-0.6 million
MAN 1-2 million
2 hour rail:
LPL 2-3 million
MAN 6-7 million
Since that report Liverpool South Parkway (10 minutes by shuttle bus to LPL) has been built and has direct rail services to Norwich for example.
Having said all that they did predict that by 2015 the passenger throughput forecast for LPL (2008 5.3million) ranged from 3.6-5.8 million and MAN (2008 21.2million) is 34.8-42.3 million.