In reply to Alex Pryor:
Yes, 2 1/2 hrs sounds about right. Fortunately (but irritatingly), the rain stopped during the descent and the waterfalls cleared quickly, allowing a relatively dry descent. However, the claps of thunder around the summit, some quickly developing towers of cumulus, and return of rain as we neared terra-firma confirmed we'd made the right decision. The lower slabs were actually OK as the diagonal trough you describe seemed to channel the drainage off rightwards towards the line of Durendalle.
We were a bit gutted to be robbed of such a quality route at that stage, yet perhaps lucky that the storms started when they did as we didn't fancy being cooked for the vultures on the final ridge with no easy descent after pitch 18.
Nevertheless, an outstanding day of climbing on wonderful rock on a stunning face. I look forward to the re-match.