Met Office has the May summary data available, and I prepared these images for our own Scotland May'13 trip report (coming soon). Thought you all might like to see them.
Left images are actual, and right images show Anamoly.
It felt awfully darn cloudy on our trip, but I see that 2013 May hours of sunshine was about average for May - averaging about 5 hours a day. Given that on May 15 in Torridon there are 16 hours of potential sunshine, the normal of 5 hours a day is, well, not like California in May.
Rainfall in the western part of Scotland was above average - in the 125-175% of normal for the month in most areas.
And finally, while we were hiking many locals told us that it was an unusually cold spring. When I look at the data though, May is not much cooler than usual. March and April mean temps were a couple degrees C lower than normal, but May temps are only slightly below normal in some areas, and close to normal in others.
These graphs, and many others, available here: http://www.metoffice.gov.uk/climate/uk/anomacts/