What tubby describes is more or less correct.
All the LCCs do this. Buying new aeroplanes at a huge discount and then immediately refinancing (sale and leaseback or mortgage) is how the large LCCs make money.
You think that they made all that cash charging £19.99 a ticket? Nope it is from buying aeroplanes. The flaw with this model is the constant requirement for expansion. Eventually the bubble will burst (but people have been saying that for 15 years and it hasn't yet).
This is exactly the model that Monarch were going to pursue once the old jets had left the fleet and is the concept that the owners bought into in 2014. Unfortunately the transition could not be made due to severe competition...blah blah blah.