Hi Gavin,
Without calling your methods into question, I found the opposite to be the case for May this year. I had a look at the stats for my local weather station (Royston,Herts) and I noticed that the winds were very much coming from a northerly or easterly quarter for most of the month so I checked out the synoptics for May and saw for around 2/3rds of the month, there was high pressure over greenland or the mid atlantic causing the jet-stream to head further south forcing the storm track over the channel, thus giving us the appauling month we had. Wouldn't that suggest we had a neutral or negative phase of the NAO for the month? I could be reading this completely wrong so bear with me
Alex
P.S May 2012 looked incredibly similiar to 2008,9 and 10