Well you're probably in the wrong forum, but the BA forum does have a EU261 compensation thread in it (see link in the Dashboard), and that will guide you as to what to do. Just read the first few posts plus some of the last posts. You would need to prove that either that the cause of the delay was under Qatar's control OR that they did not do all they reasonably could to avoid the delay.

Because it was leaving the EU you are covered in that direction, in the reverse direction you would have no cover since Qatar isn't a European carrier, so that's worth bearing in mind when choosing flights.