I have bought a hob that has a standard half inch gas inlet but does mean I cannot use the existing 10mm gas feed without having to have the pipe replaced by 15mm gas feed piping

Pipe thread is different than a machine thread. Even in some European countries, they use standard american pipe thread which was derived from the British system. You may not have to replace the entire line, but you will have to size up the 10mm pipe. You may find a bushing adapter that would thread into the hob and then the supply pipe will thread into that. One of my sources show a 3/8 to 1/2 bushing adapter but in parentheses it also states (10mm to 15mm). I'd check a good hardware store. Make sure you use the appropriate pipe sealing material.