From the dealership or can you buy it from Halfords etc? I need to top up and part way through an extended tropical and haven't properly researched it yet....

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Halfords do it, most petrol stations I've been to seem to sell it too if you're caught short. 

Even sold in Tesco's.

Go to a farm that has a modern tractor. They will have some. Just chose the one with the friendly farmer.

Since looking at Adblue Tesco i have pop ups for 10, 20, 40 litres at good prices.

 Amazon and free delivery.