Sainsburys forecourts seem to have this deal nationwide, i visited one last week in Derby and have just found it in Gloucester today.
600ml Car Plan Blue Star deicer only 74p
In comparision they had the 300ml next to it for £1.99
Crazy deal as places like Tesco are selling 600ml for £2.50
I cant see this online so must be in their forecourts only.