Depends, BA may be correct here. If you look upthread, you will notice that BA (and most airlines) treat long -> short flights as if it was the short flight was cancelled, if the problem was solely on the short leg. As also indicated upthread, this is open to legal debate, nevertheless this is what BA does in reality. So on the face of it, they have paid the right amount here (given their normal approach) if the long flight was more-or-less ok. However, if the delay offloading in LHR meant that you would anyway have missed your connection and been more than 3 hours late thereby, then it's the 400€ limit that should apply. If there was a long connection time anyway then you are on the lower amount. If this is looking a bit murky, feel free to put your scheduled and actual timings down, there are quite a lot of variables here.