A £5 rise in fuel costs on a £100 ticket you have a problem. This is worst if the same fuel cost rise is on a £40 ticket. This is why planes are being rested in the winter.
For ryanair the big problem is the effect on the 2nd hand plane market. Ryanair has been used to buying new planes at close to cost and selling them on before they need major work at close to the price paid.