I dug out an old impact driver I obtained years ago. It's got assorted bits. I propped a piece of thick softwood between the Carbs and the frame and used the impact driver. screw released quite quickly! It surprised me because the head was really mashed up.
The No2 impact driver I bought yesterday has been already dispatched so that's another tool I probably won't ever use
I'll have a garage sale before I move to a smaller place
Going to London for a long weekend so I'll continue the carbs saga next week