If you are not successful finding the proper part, you may want to go to McMaster-Carr (mcmaster.com) and search for the keyword "pipe grommet". The smallest pipe grommet has a through-hole a bit over 5/16" and fits a 1-1/16" panel hole. I know they are not original but I have used them for things like wiper rack tube grommets for other British cars. They can be stretched a lot. If they cannot stretch enough for this application you could split one with a razor blade, place it over the capillary tube, then Krazy-Glue the joint back together. Of course, McMaster also has traditional grommets but you may find you have to buy a whole bag to get the one piece you need.