and say that you want to travel from Oxford to Hawarden at 8am tomorrow. (I'm not sure how relevant the time is, and I think any day will do.) When it comes up with the results, click on the "Check fares" button. This produces the following results:
The cheapest available fare is: £179,769,313,486,231,570,000,000,00
Presumably this is a basic ticket, because there are no first class tickets available for that journey. Personally, I'd expect to at least be guaranteed a seat if I was spending a bazillion pounds on a ticket! (I haven't counted the zeros, so I'm not sure what the correct terminology is for that power of 10.)
Source: I swiped this from the "BackBytes" column in the latest issue of Computing, but I can't find the corresponding article on their website to link to directly. I've reported the problem to National Rail via their website, on the basis that it's a bit mean just to laugh at them, so you may get different results if you repeat this search later.