Am I likely to get a cheaper flight with a long layover?

It's not the length of the layover that matters. Whether you just change planes in the intermediate city or stick around a while shouldn't make much difference, or possibly it would cost you more to stick around a while.

What you're hoping to find is an unpopular city B (relative to the flight capacity in and out) so that flights A-B, B-C are cheaper than flights A-C. This is generally the case when A and C are wildly popular places like London and New York. It's less likely to be the case when A or C is already kind of a backwater. Of course, if A and C are TOO popular then everyone has capacity between them and there may be no room to save money.

When I was looking for flights from Toronto to Amsterdam (YYZ-AMS) I recall it was about a 30% saving to go through Iceland. In the end I valued my time higher and took a direct flight. For a recent conference, routing Toronto-Sofia (YYZ-SOF) through Istanbul instead of the usual European hubs (Paris CDG, Frankfurt, etc) cut the price in half.

I think in your case there is no substitute for searching. Any "find a flight" website will offer you both direct and changing-planes versions of your flights: Expedia, Kayak, whatever the cool kids use these days. If it starts offering you one-stop flights that are the same price or slightly cheaper than the direct, go to that airline's web site and try pricing a multi-city with a delay in B, or call that airline and ask if that would affect the price.

Generally speaking most international point-to-point flights will not offer any savings when compared to transit flights. The only possible exception would be to find a cheap flight from a different European city and then using a low-cost airline (such as Ryanair or Easyjet) to get there.

For example, I managed to find a flight from Oslo to Newark for 430 GBP. You could theoretically buy a ticket from Stansted to Oslo through Ryanair and then take the flight to Newark.

However, you should also note that saving 100 GBP won't always lead to actual savings as accommodation, transportation to/from the city center, food, etc, can often cost a lot more than that.

If you are looking for a long lay-over, you might look at booking two flights, rather than just one. In some cases, this can save money, if one airline offers the cheapest fare between A and B, and another offers the cheapest fare between B and C.

Be sure to leave a sufficiently long layover each direction so that if your first flight is delayed, you won't be late for your second flight. This is always a risk when booking two separate flights.

I did something similar a few months ago for a one-way flight from Mexico City to Paris. By booking two one-way flights--the first from Mexico City to Montreal on AeroMexico, the second from Montreal to Paris on Condor with a 12-hour layover between. By booking two one-way flights I was able to save about 45% versus a single airline.

Your mileage will vary.

Quite often the cheapest option is an airline that has a hub in a country that is neither the origin nor destination, so you have one flight inbound to their hub and one outbound from it on each way.

As expected, the cheapest flights I can see (using random dates in April) are

• Aer Lingus via DUB
• Air France via CDG
• Air Canada via YYZ

I would have expected Iberia (via either MAD or MEX) to be on that list as well.

Air Lingus is a budget airline, so I'd avoid that unless I know that none of their surcharges will stick to me.

The price difference for the first direct flight is not that great though, so I'd weigh against the cost of the layover (either airport food, or a trip to the city) here.

@CSJC I built a tool to help you find exactly what you're looking for here: www.cleverlayover.com

It will search all A->B + B->C combinations to find a cheaper way to get from A->C. Looking into the data, it is actually quite effective: 33% of searches have some form of savings, and the savings average > $300 when they exist. Try it out and let me know what you think!