Can somebody please explain this to me?!
I just bought a business class ticket with Qatar from Cardiff to Seoul via Doha. It cost Ł2058 via SkyScanner. Out of curiosity I later searched SkyScanner again for a business class return from Cardiff to Doha on the same flight (just the first leg of the ticket I purchased). came in at Ł3300!,,, Yet my journey still costs the same on SkyScanner despite being significantly longer. HOW CAN THIS BE?

Because that's the way it is.

Airline fare pricing is complex, and takes into account any number of factors. It's not as simple as "a 500 mile flight will always cost XXX and a 2,000 mile flight will always cost YYY."

This is a typical and well known issue with Qatar Airways - and many other airlines when you originate or end Travel at their hub. Flying QR to Doha can be quite a bit more expensive than flying on to India, for example. Flying AA out of DFE, rather than say AUS, IAH, SAT can cost significantly more as well. The Airlines establish tariffs they consider will render the best profit.

Welcome to FT!

It's all about competition and making money.

Someone who wants to get from Cardiff to Seoul has many options, most of which do not connect in Doha, and some of which connect via places such as India where flights tend to have low prices per unit of distance. That means that competition has a significant effect on prices. Someone who wants to get to or from Doha, on the other hand, will find a more captive market with fewer options. That leads to a phenomenon that has been documented many times: flights to/from captive hubs cost more. (Compare, for example, the fares of Delta flights to/from Atlanta with its fares on other itineraries that happen to connect through Atlanta. Its JFK-ATL-LAX tickets have to compete with JFK-LAX non-stops, but someone in JFK who wants to be in ATL doesn't have many other options.)

There is, of course, nothing to stop someone who wants to go to Doha from buying a ticket to (for example) Seoul and leaving the airport at Doha instead of boarding the final leg - except three things:

1. That passenger's checked bags will, in all likelihood, end up in Seoul. Under those circumstances, the airline might not want to fly them back at no charge.

2. As soon as that passenger's Seoul flights leaves Doha without him or her on board, the rest of the ticket - including the return flight - will be cancelled.

3. If any of the early flights doesn't operate for any reason, Qatar Airways will quite possibly put him or her on board another airline's flights to Seoul. Those flights are unlikely to come anywhere near Doha.

These factors combine to keep use of this "hidden city" money-saving trick down to a level the airlines can live with.

Supply and demand

Because many people will prefer to fly LHR-ICN directly, not stopping in DOH. So to pick up some traffic, QR provides a competitive fare.

And supply and demand.

One reason is more competition on the Cardiff to Seoul route (when you take all the possible routings into effect) than on the Cardiff to Doha route (where it's unlikely there's much if any competition).

Routes that have more competition often have lower prices than routes with little to no competition.