My husband put a fuel cleaner (Upper Cylinder Lubricant and injector Cleaner - Lucas brand) into the diesel tank, then filled up completely with diesel. Immediately after running engine, a light came on. There were two error lights - one was the DPF light, which he reset and is now off; the second light refers to a P2585 error, and the accompanying message on the computer is 'Fuel additive control module warning lamp request'. When he attempted to reset the light, it did go off, but turned back on immediately after starting the engine again. It now remains on.

Any explanations / suggestions? Is it safe to drive the car? What repairs / changes need to be made?

This code is possiby referring to the DPF system, which requires an additive to run correctly. Quite often overlooked as many people aren't even aware they have them. To be safe get it checked out as soon as possible as if the actual DPF goes, it will be an expensive replacement...

you need your EOYLS tank topping up and resetting. it should be topped up and reset every 3/4th service and the DPF replaced every 90/100,000 miles