Complex sentence: Trident would replace the previous system of four Resolution-class submarines each carrying 16 Polaris A3 missiles (known as A3T) with 3 ET.317 warheads (not MIRV ed), latter upgraded by the Chevaline programme to A3TK with a limited MIRV capability.
Split into simpler sentences: Trident would replace the previous system of four Resolution-class submarines, each carrying 16 Polaris A3 missiles (known as A3T). These were originally armed with non-MIRVed ET.317 warheads, but were latter upgraded by the Chevaline programme to A3TK with a limited MIRV capability.