Skoda Yeti gearbox

[V8s ONLY]

Yeti 1.6 diesel greenline.18 months old,8k miles,has started to jump out of 5th gear.The wife,whose car it is complained it would not go in 5th sometimes,I did not experience this until a couple of days ago,and would say it goes in but immediately jumps out again . Anyone know if this is big problem or just an adjustment? It should be covered by the warranty but we bought this car very cheap as a trade deal,I think it was an ex motability car,and have had problems in the past with main dealers when cars are not bought through normal channels.