The most important thing you can do right now is ask questions and take notes, especially regarding what the Thomson reps say regarding your flight (and get their names if you can).
What has/do Thomson said has caused the flight to be cancelled for the day/delayed?
Is the aircraft on the ground at Goa? If so when did it arrive?
Based on your (albeit limited) information and that of barbexpat, it would appear that you have a valid claim for 600 Euros each for either flight cancellation or delay. BUT, the airline (who would rather not have to part with that money) may well do all it can to deny any claim and cite "extraordinary circumstances". That's where your notes will come in handy, as any airline representative who lets slip that the delay is caused by a mechanical problem will help your case. Mechanical problems usually don't count as an extraordinary circumstance.