In James Cordens biography, he said they spent every single day together for three years including Christmas day etc then when Gavin and Stacey finished, they never spoke again.
Apparently Matthew is quite difficult. His take was that when he plays a character, for the whole time is he that character and will live as that character so when he finished as Gavin, he had no need to carry on living as h and that it was the end of the chapter of his life so he no longer had a need to be friends with James.
Quite sad really as it seems James valued the friendship more than Matthew did.