I have been looking into Martin Carthy's tuning and string gauges lately, and I managed to find this note on the subject from Roger Bucknall of Fylde Guitars, who has worked on Carthy's guitar a lot over the years:

"He uses custom string sets to accommodate his favored tuning (C G C D G A, low to high), with typical gauges (high to low) .015, .018, .027, .039, .049, and .059."

This was a quote from Acoustic Guitar Magazine, July 2005, No. 151. The complete quote can be found here:

http://editthis.info/geartome/Martin_Carthy

cheers