Following on many earlier threads, I was at the Bristol Makros last night. Their cheapest SVO was =A314.99 for 20L and zero VAT. I got 120L which at a 50/50 mix through the winter will do me for 4 fill ups. I'll move to a higher concentration:
(a) When I get the heater fitted; or (b) When I get the valve and dual tank system working; or (c) When it gets warmer!
I don't think that is much cheaper than you can get at Tescos/ Sainsburys et al, but coming out of Tesco's with a 120L of SVO on a trolley may cause a few eyebrows to be raised (assuming they will let you purchase that amount).
Cheers
Peter