Hi, i have this pipe and connection fitting on my radiator upstairs which i want to move, make it longer, so i have bought some GPSPEEDFIT pipe and fitting, o and a pipe cutter, proper one, when i come back from buying what i needed, the pipe looked a little bigger, weather this is internal and external, i'm not sure, but measuring the speedfit pipe which is 15mm to the pipe in the picture, the pipe in the picture is bigger by a couple of mm, so is this the thickness of the pipe? outer? or inner as well, i can't see any marking on this pipe, that tells you the size of it, i want to connect it straight to speedfit connection to extend it, along with the other pipe, thanks