Hopefully i can help..
Ok..normally missfulling diesels is about 5 litres is about the max you can put in by mistake to be able to risk topping the tank right up with diesel.however normally its best practice to actually drain the tank - because you have 8-9 litres then normally thats means a full system clear out of fuel lines / tank emptied / and a new fuel filter before adding fresh diesel into the tank. if as in your case the engine has been run then the entire high pressure fuel system is supposed to be replaced. This is the manufacturers advice.financially the cost runs into thousands of pounds.
From experience you have done the best thing you could have in the circumstances, If it where me or advised a one of my customers at the moment i would recommend having the tank drained and the low pressure lines cleaned plus a new fuel filter. The problem with leaving the petrol in is that if it does cause a fuel system failure then it wouldn't be covered by any warranty on the car. Petrol is what you class as a solvent and washes diesel out of the fuel system lines, the diesel is an oil so acts as a lubricant for the fuel pump and other moving parts of the fuel system including injectors - and from what you have mentioned with the hard starting the damage "could" be in the stages of failure..If you want to save the car or hopefully the injection system/fuel pump..you need to find a local company like of the AA or any certified miss fuelling extraction firm to come out the car and drain the the tank out clear the lines and replace the fuel filter...I do hope this helps