If your flight was delayed by more than 3 hours, you are entitled by EU law (Regulation 261/2004) to claim up to £510* from the airline depending on the distance of your flight.
Up to 1,500km
1,500km to 3,500km
£210† / €250
£340† / €400
£510† / €600
The amount you receive is irespective of your ticket cost and is set by EU Regulation 261/2004.
† Your claim will be settled at the Euro equivalent on the exchange rate at the time of the delay, or on the date your claim is settled (This will depend on the airline).
‡ Between an EU and a non-EU airport.