People - many thanks for the informative posts to my original question, but it's possible that general discussion of the often seen habit of buying a cheap return ticket and not using the 2nd sector is probably best discussed in a thread of its own.
Does anyone have any comments particularly relevant to the London-Madrid route, and IAG's pricing policy ? I'm particularly puzzled as to why there are cheap-ish one-way fares on LCY-MAD but the only one-way tickets for LHR-MAD are in business class, particularly given the existence of flights with Easyjet / Ryanair from Gatwick, Luton and Stansted. I can't figure out what kind of market segmentation IAG is trying to achieve by this policy.