Had to fill ours up the other day, B&Q are doing 10l for £12 which seems to be about the best price (saw some on a garage forecourt - which you shouldn't touch - at over £30 for 10l

). As a bonus, the pouring spout has a decent seal in it and actually works(*)! This seems to be a long term price as it was the same a few months ago when my Dad had to fill his up.
* This is especially important on our 2017 C4 Grand where the filler is in between the second and third row of seats effectively in a hole, and is pretty difficult to get to!