All becomes clear, a Halfords banner ad and a scrolling side ad now appear on the Guardian page when I open it. So it is a word of mouth campaign aimed at an 'urban' audience, the bikes aren't intended to be widely available.
I think that's down to tracking cookies (aka 3rd party cookie) and targeted advertising.
Visit Halfords website; Halfords puts one cookie on your computer, marketing company puts one cookie on your computer.
Visit Guardian website; the same marketing company reads their cookie already on your computer and finds out you earlier had looked at the Halfords website, then puts the Halfords advert on the Guardian web page you see, especially for you.