Complex sentence: The 75th went so far as commissioning a marble monument to themselves in Malta where they were based at the time, and had spent so long in Dorset that they were effectively an English unit that had to learn how to wear kilts.
Split into simpler sentences: The 75th went so far as commissioning a marble monument to themselves in Malta where they were based at the time. They had spent so long away from Scotland that they were effectively an English unit that had to relearn how to wear kilts.