With an '87 I’m presuming it’s a Lucas CEI. And I’m presuming you have spark, since you’re a mechanic and would check that first.
I’m gonna go farther and presume that the Daimler has the same little coax wire that the XJ-S has. So, follow me on this: The ignition amp is a black box on top of the LH intake manifold and it says AB14 on it. From that box, there are wires that go to the coil, wires that go to the distributor, and two wires that go rearward toward the firewall. The two going toward the firewall are blue/white/gray and both end in bullet connectors, but one is a male end and one is female. One of the two, after the bullet connector, becomes a slim white wire and immediately enters a wiring harness, wrapped in a black loom, and then crosses over the rear of the engine to the RH side where it merges with a larger harness. The skinny white wire is the only wire in the loom for that crossover to the other side.
Strip the loom off that little harness portion and inspect the little white wire. That white wire is actually a coaxial wire, meaning it has a grounded shield and a core. With age and heat it gets brittle and cracks and the core shorts out with the shield and the injectors stop. If it looks bad, strip out that 18 inches of coax and replace.