Driving home last night, it died on me. It was pulled out of a junction, got into 3rd when the revs went high for 5 secs before going back to normal. Then it got into 5th and revved high and smoke started coming from the back. I turned off the ignition and kept revving. There was black smoke from the back and white steam/smoke from the bonnet. She stopped and now won’t turn back on.
Being a diesel and that the engine ran for a minute with the ignition turned off, makes it highly likely that you experienced diesel runaway.
During normal operation, diesel is used as fuel for the engine and turning the ignition switch off cuts the supply of diesel.
If an engine oil leak occurs where the leaking oil is sucked into the air intake of the engine, then the oil will act as a fuel for the engine instead of the diesel. The most likely place where the oil can leak from is the turbocharger.
Since you are in no control of this oil supply, the engine revs uncontrollably until the oil is used up, both starving the engine of lubricant and over-revving the engine.
It is highly likely that the engine is now wrecked and will need replacing, but there is a slim chance that it is reparable, so get it checked by a mechanic.