OK with a modern diesel engine you can get away with about 15% petrol in diesel with no ill effects
as its a 70 litre tank then 1 litre is only 1.5% so you will absolutely fine
If you want to be absolutely sure then I'd say to add a bottle fuel treatment such as this
that will add a does of lubricant into the fuel and help protect the pump seals against any damage from the petrol
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