Thomson will almost certainly want to see your boarding pass, so hope you still have it. Probably also useful, if you still have them, to keep the luggage tags that were attached to your luggage when you checked in. Anything that can prove you were actually on the affected flight is good...
I agree with happypig above. Airlines are no longer allowed, as they once were (and still try very hard) to allow technical reasons as grounds for not paying compensation. If your flight arrives at its destination more than four hours later that it was scheduled to, then compensation kicks in...