Fitted a new air filter which arrived today. I have now found what the hissing sound was under the bonnet. Turbo hose split. Tried to repair it with super glue until the new hoses arrive which I have ordered.
The glue seems to work, it has stopped the black smoke when under load.
Nothing on Disco today. But I replaced the rear gear shifter on my bike, only to fail in adjusting it for tomorrows ride.