Route 1:
Train from Alton to Waterloo.
Northern Line to Leicester Square
Piccadilly Line to St Pancras.
Train to Luton Airport Parkway.
Shuttle bus to airport.
Typical total journey time is just under 3 hours.
Return fare = �47.00 (which includes the underground and shuttle bus)
Route 2:
Train from Haslemere to Waterloo.
Then as above.
Typical journey times are 10 minutes quicker, but the fare is much dearer (�56.80 return).
Links:
http://www.london-luton.co.uk/en/airport/
http://www.nationalrail.co.uk/
Chris