Well, I "bit the bullet" and bought 400 litres of oil at €330, which works out at €0.825 per litre.
I was reluctant - not because it evaporates (it does, but not much) but because my plastic 1000 litre tank is ten years old and I'm worried that its useful end-of-life might be approaching. I plan to buy some ratchet straps, when (if) the shops open, to reinforce it. Also need to build a wall on two sides to shade it from sunlight. Better late than never.