Many airlines try all kinds of different tactics to refuse to pay the financial compensation that passengers are entitled to when their flight is delayed.
For example, Thomson is telling passengers the period of limitation is 2 years based on the Montreal Convention. Many passengers do not know that, in fact, they can claim for flights dating back 6 years when they depart or arrive in the UK. This is exactly what airlines try to achieve; confusion, in the hopes that passengers give up.
Luckily a British Court put an end to this. The judge ruled in favor of the passenger and decided the 6 years limitation is applicable in the UK.