It was all my fault. When it arrived from Italy, the W/NG had a 276 fitted and I renewed the needle jet for the 276 because the one in it was knackered, and all was well. Then I found a really good 275 body, marked for a W/NG and I swapped the needle jet and main jet into it with a new slide and needle. Of course, the 276 uses a 107 needle jet and the 275 needs a 106 so I had unwittingly installed the wrong one, so finding that improved the situation.
It was a good run though and she was humming along getting to 50 easily on the flat, though of course I am being very careful not to stress the newly rebuilt engine.