Complex sentence: Returning to the "bomb factory" in Glasgow they modified the Jeep into an improvised bomb during which time police were tracking both suspects via the mobile phones they had used to try and detonate the London devices and which the men were still carrying.
Split into simpler sentences: Returning to the "bomb factory" in Glasgow they modified the Jeep into an improvised bomb. Police were tracking both suspects via the mobile phones they had used to try and detonate the London devices and which the men were still carrying.