Originally Posted by Mizter T

Hmm, haven't noticed that before - AFAIK, none of the other airline affiliate portals for the either the Stansted Express or Gatwick Express ask for this.

There's actually nothing in the ticket's terms & conditions which restrict its usage to Ryanair passengers - so I think this is nothing other than a little obstacle they've put in the way to try and dissuade everyone and anyone from buying these cheaper tickets.

For what it's worth, the little pop up message that appears when you hover the cursor over the yellow question mark icon on the payment page (or doesn't, if using Firefox!) says this:

To obtain the Ryanair discounted price, please enter the 6 digit alpha/numeric reservation number as supplied on your Ryanair flight confirmation email. Please note this reservation number can only be used once.

I'd just put in a spoof PNR here - by the looks of things, all it might do is block you from using that same PNR for another purchase for a period of time (how long does it take for a PNR to be recycled?) - either that, or perhaps it can identify whether the given code is in the correct format to be a genuine Ryanair PNR or not - I'm no expert on PNRs so perhaps others can step in with some wise words here...

I would however think it very unlikely that there's any backend link-up between this basic rail ticket purchasing system and Ryanair's database whatsoever.

I would also very much doubt that the PNR given would be printed on the rail ticket itself - rail tickets here in Britian are credit-card sized, and only have a limited number of fields - furthermore the on-train ticket inspectors simply wouldn't be in the business of trying to match discounted rail tickets to boarding passes.

Lastly, one doesn't absolutely have to have pre-printed one's boarding pass to fly on Ryanair - sure, if you hadn't they'd charge you a £/€40 "Airport Boarding Card Re-issue Fee" - but inevitably some people will have nonetheless managed to forget or lose their print out, and others may have found their printer decided not to work/ run out of ink/ run out of paper at the last minute, or else been befallen by some other calamity!