Thank you and this is a very good idea. I will look out for this option in the future.

However, in this particular case, it is slightly complicated. This is actually for our daughter who comes on a two-week holiday in Hong Kong. We want to take this opportunity to visit Seoul together. In order to minimise wait time, my plan is for her to fly LHR->HKG then leave her big suitcase and stay overnight, then HKG->ICN the following day. After staying in ICN for 4 days, she would use the same set of tickets for ICN->HKG, then stay for 8-9 days before flying back HKG->LHR. My wife and I would just get a set of return tickets matching her HKG->ICN and ICN->HKG. My consideration is (1) protect the HKG->ICN in the event of delay on LHR->HKG and (2) obtain the cheapest/most convenient combination. I might have overthought this but is the solution to buy two three separate tickets for one LHR->ICN with longest stopover selectable, one ICN->HKG and one HKG->LHR (but this combination means she will have to drag her big suitcase in Seoul)?