Complex sentence: Dinky offerings at this time were striking, but due to the lack of much competition, development of new models was perhaps a bit slow at least until July 1956 when Mettoy introduced a rival line of models under the Corgi brand name.
Split into simpler sentences: Dinky offerings at this time were striking, but due to the lack of much competition, development of new models was perhaps a bit slow. That was until July 1956 when Mettoy introduced a rival line of models under the Corgi brand name.