Are you referring to the Harpenden - Heathrow fares routed RAILAIR STALBANS? This allows you to take the train from Harpenden to St Albans and then catch the Green Lines 724 bus from St Albans to Heathrow, all on one ticket.
If you were departing from a station on FCC's Great Northern Route you could do the same thing changing at Hatfield (route RAILAIR HATFIELD).
There is a bit of information about these tickets in this thread:
http://www.railforums.co.uk/showthread.php?t=24329#4
But I have never seen this service advertised publicly anywhere so I don't know if these tickets are actually sold in real life.