Stansted to Luton Airport?
 

Online it says there is a NationalExpress shuttle service. However, the website never works to buy any tickets. Can you do this at the airport?

Yes

Thanks. We have about 6 hours before the next flight. Is that plenty of time?

Normally it would be - but note the service isn't very frequent (2 hour gap between services at 9:15am for example) - so the wrong combination could cut that 6 hours to less than 4 and there's always the possibility that the first coach is fully booked, so in extremis you might have to throw money at the problem and take the Stansted Express to Tottenham Hale, Tube to Kings Cross then train to Luton Airport Parkway from either St Pancras (next door to KX) or Kings Cross Thameslink (200 yard walk from KX). See www.nationalrail.co.uk for train times

Why can't you book tickets?

The national express site is always down for me. When I got to teh form to buy tickets I hit the button and it sat there for 10 minutes.