Also dirk85 unless you have a statement from your CAA to the effect that they agree then just cos it's written in your OM doesn't mean it's ok - it means that it might be OK but equally it might mean that no one has noticed up to now.
The OM has been formally approved by the authority, whether they made the effort to actually read that chapter or not is not my business: as long as they did not openly disagree on that, and the CAA stamp is there, we are legal to follow what is written.
For the rest it is good common sense not to fly there with heavy rain if you are dubious about the runway conditions, grooved or not.
But we were talking of legality here...