Because I couldn't get it to run perfectly, I ended up going to ATSpeed Racing, a local rolling road/Weber specialist.
Although he made a really good job of balancing the carbs and it did make a difference to the running, he was on the phone to Weber to try and sort out the idle jetting. No matter how big a jet he tried (I think he ended up with a 70 in as I recall), it was still running lean. His "Guru" at Weber identified the carbs as being made specifically for Alfa Romeo and apparently these have large progression holes, which causes this issue at idle and low revs.
He said these would be fine for racing, because of how hard you rev and, interestingly, it was originally built as a sprint car.
Bottom line, he could fudge it to make it OK, but was almost embarrassed that he couldn't do a proper job for me.
He said ideally I would fit a new pair of 151s and get them jetted so, with one birthday present outstanding, I negotiated with Mrs T that instead of a nice watch to remind me that I have turned 60, I would order a nice shiny pair or carbs to make me feel 20!