Complex sentence: The original plan was to replace the Royal Arsenal's Filling Factory with one at ROF Chorley and another at ROF Bridgend, but it was soon realised that many more ROFs would be needed.
Split into simpler sentences: The original plan was to replace the Royal Arsenal's Filling Factory with one at ROF Chorley and one at ROF Bridgend. It was then realised that many more ROFs would be needed.