Complex sentence: Guano has been taken off the island regularly and, for example, in 1880 and 1881 Ichaboe Guano was claimed to be the "richest and most fertilising guano imported into England" and contained 12 to 13% ammonia and 27 to 30% guano phosphates.
Split into simpler sentences: Guano has been taken off the island regularly since then and, for example, in 1880 and 1881 Ichaboe Guano was still being sold. It was claimed to be the "richest and most fertilising guano imported into England" and contained 12 to 13% ammonia and 27 to 30% guano phosphates.