Complex sentence: They had left the van unattended in the carpark which was the start and finish point for the run and it was there that an IRA Active Service Unit (ASU), who had been following the van, placed a booby-trap bomb underneath the vehicle.
Split into simpler sentences: They had left the van unattended in the carpark, which was the start and finish point for the run. It was there that an IRA Active Service Unit (ASU), who had been following the van, hid a booby-trap bomb underneath the vehicle.