Hassid came to London with his father in 1938 at Flesch's invitation, to continue studies with him. Flesch concentrated on his musical and interpretative development rather than technical skills. Musical celebrities who heard him play at Flesch's house and were astonished at his ability included Joseph Szigeti, Jacques Thibaud, David Oistrakh and Fritz Kreisler. In a passage supplementing his father's memoirs Carl F. Flesch wrote that "Hassid was no doubt one of the strongest violin talents of his time. Indeed Fritz Kreisler, after hearing him at my father's house, said: 'A fiddler like Heifetz is born every 100 years; one like Hassid every 200 years.'" Kreisler lent Hassid for the remainder of his career a violin of 1860 by the French maker Jean-Baptiste Vuillaume, which was a great improvement on the instrument he had played up until then.