I bow to your greater expertise on the subject, Sir George.
But if I understand you correctly, you are saying that Marshall Asphalt (the top 'coat') does leech materials that when combined with water result in a slippery surface. The runway was being progressively resurfaced and so, although there was an area with exposed regulating course, as the project progressed there was an increasing length of the newly surface runway that was finished with Marshall Asphalt along with the associated potentially slippery surface. This, I understood to be a principal cause of the problems. Strong crosswinds are nothing out of the ordinary art Lulsgate Bottom but the amount of standing water may have been increased because the 'drainage' mechanism was not complete. Whatever, these two points will have contributed elements to the incidents - holes in the cheese as Jim Reason would have it - perhaps if the resurfacing work had been handled in some other way one more hole wouldn't have lined up.
I'm not sure that the runway not being grooved is completely irrelevant. If the runway was designed to have a grooved surface (as I believe it was), I presume it would have been profiled to enable the surface water to run off through the grooves. As I suggested a moment ago, if the grooves were not there the drainage properties would not have met the design criteria. It's interesting that you mention a selection of other runways that are not grooved - it does surprise me because I was pretty sure that Manchester and Bournemouth were both grooved but, again, you're probably far more up to date than I. But are you saying that grooving a runway makes no difference to its friction characteristics?
As to CAP 683, whilst I haven't looked at the latest re-write in any detail I fully understand that it's essential to know what CFMD was used to make the measurements. But if I recall correctly (and they haven't changed it) CAP683 is about measuring friction with a self-wetting device (i.e. a known volume/depth of water) for the purposes of planning when and where maintenance work is required rather than environmental measurements for operational purposes. Whether this is the case or not, it is true that one needed to know about the CFMD used to generate braking action values - and I think this was an essential part of the information that used to be provided when the data were provided. I think the old way of providing the data on request was intended to enable operators' Ops Depts to investigate the runway conditions and CFMD used at specific airports in good time and to be able to make informed judgements (and perhaps via guidance in the Ops Manual) regarding the use of the runway in adverse conditions. But I wonder if this ever happened....