Flight Delay And Compensation

[KARL]

Regarding the EU law on Air Passenger Rights, does anyone know how long an airline can claim delays to schedules are "beyond their control" ? Can they deny responsibility to compensate (as the law generally stipulates regarding delays) on the outgoing flight because the incoming aircraft to serve it was delayed by weather and subsequent crew duty time limitations) ? Can they then continue to do so in a chain of further subsequent flights ? I can't find any online source, such as the CAA, where I can ask the question and expect an answer (only on specifically quoted UK originating flights, the one in question arrived in the UK) - any suggestions ?