We did the same, moving to the Langley Park catchment. It gave us a headache though. The LP school is so good we were undecided right up to the last as to whether to put that as first priority or the grammar school that my son had got into. We eventually decided on the GS.
Upside: our younger son also has the comfort of knowing he will get into a good school.
Downside: our first son's journey to GS would actually have been considerably easier from our old house!
In which GS is your ds now - Olaves? Asking as we will apply there as well, however considering how selecive is, do not really think he will get there.