Complex sentence: Only three examples (the original plus successive prototypes Type 457 and Type 461) were built in total due to refinements in the existing Lancaster bomber rendering it suitable for the role for which the Windsor had been designed.
Split into simpler sentences: Only three examples (the original plus successive prototypes Type 457 and Type 461) were built in total. This was due to refinements in the existing Lancaster bomber, rendering it suitable for the role for which the Windsor had been designed.