The Hadley Centre climate model HadCM3 (the UM) requires the carbon dioxide and other trace gases to be inputted as the mass mixing ratio (MR) in kg/kg. That’s:
MR =ppm(gas)/ (mol. weight(dry air)/mol. weight(gas) ) /10^6
If like me, opening excel and looking up weights is a pain you don’t need, here’s a quick calculator to convert carbon dioxide in parts per million to the mass mixing ratio, and back again. Further gases may be added if I get time/inclination/asked. (Molecular mass taken from here.)