Is anyone here familiar with Tom Bryant of Wiscasset, Maine? He has a blog: thosbryant.wordpress.com. I found his blog while researching rebuild options due to my H6 carbs having excessive throttle shaft slop. If you search on his blog using the keywords California and 122, it’ll bring up a start to finish entry on how he rebuilds carb bodies using delrin/ acetal bushings and original size throttle shafts. As SU-specific carb rebuilders here in the San Francisco bay area are now almost non-existent, I took up the challenge and can say I’m really glad I did. I checked my shafts in their now rebushed bores and my dial indicator shows 0.002" total lateral play. This, after having spent some four hours off-and-on in front of a vertical mill (a drill press is also fine), and some $75 in tools and materials inclusive of tax and shipping. This could be worthwhile for those stateside readers here as I imagine you UK readers still probably have an abundance of nearby options.
Side note: Easy-off oven cleaner is probably the most gratifying thing to watch clean old carbs. I don’t know how I missed out on this tip all these decades.