"Fenby" is Eric Fenby, a musician and composer who served as Delius's amanuensis (an assistant, often one who transcribes things--Leonhard Euler, the great mathematician, had one so he could keep setting down the theorems in his head when he went blind). The "ta, ta-ta" is Delius trying to get across a part to Fenby. It took a while for them to settle in on a way to do the job, which perhaps is why the "In B, Fenby!" sounds rather irritated. The Wikipedia page on Delius says "The [third] violin sonata incorporates the first, incomprehensible, melody that Delius had attempted to dictate to Fenby before their modus operandi had been worked out."
"Fenby" is Eric Fenby, a musician and composer who served as Delius's amanuensis (an assistant, often one who transcribes things--Leonhard Euler, the great mathematician, had one so he could keep setting down the theorems in his head when he went blind). The "ta, ta-ta" is Delius trying to get across a part to Fenby. It took a while for them to settle in on a way to do the job, which perhaps is why the "In B, Fenby!" sounds rather irritated. The Wikipedia page on Delius says "The [third] violin sonata incorporates the first, incomprehensible, melody that Delius had attempted to dictate to Fenby before their modus operandi had been worked out."