I paid £55 plus VAT minus 14% discount for an 80 litre tank (£56.76).
Since the beginning of the year I have refuelled 31 times with 2126 litres of fuel: average 68.58 litres per tank.
Assuming with a 70 litre tank I would have an average refuel of 58.58 litres, I would have refueled ~36.25 times, so have saved 5.25 refuels. Assuming each refuel takes 15 minutes, I have saved about 1 hr 15 minutes including the extra ~10 seconds for the extra 10 litres. At minimum wage of £7.50 per hour I've clawed back £9.37 of my £56.76 investment. At a rate of £45 per hour I've very nearly made my money back.
Of course, on average, I'll be carrying an extra 5 litres of fuel ~4Kg. But I'll be saving on fuel by not having to accelerate out of a filling station from stationary with a cooler engine so often. I expect the two more or less cancel each other out.