Agree, my 3er isn't a DD anymore, so its weekends only, family trips etc (no point in going for a sunday blast in the winter),
So 100% mix would be OK costwise.
If you've got space and a few cars, then why not go bulk-buy?
Xmas offer - 205 litres for £99!!
Or 1000 litres for £399!
You could sell it to the neighbours!!