This may open a can of worms but I am speaking from personal experience here so am only giving my tuppence worth. Am in no way affiliated with the following company.
In 1993 my dad brought a new Toyota Previa 2.2 petrol. Although he didn't pay for his own petrol it was still a shock to be getting around 22mpg. One of his employees did a bit of side trading with a product called Ecoflow and offered my dad one on a use now and if it works buy it basis.
My dad, who was very very sceptical, took him up on his offer and after a couple of months paid him in full. Im not going to lie and say it shot up to 50mpg but he did see an increase to 26+mpg.
I personally wouldn't pay the £60+ they want from new but they do come up on fleabay occassionally for pennies. Might be worth a shot?
As a side note, we used to sell, service and repair Ripack 2000 shrink guns which shrinkwrap plastic sheets onto pallets of whatever you have. We tried a gas Ecoflow on the pipe of this and fired it up. Without the Ecoflow you could comfortably put your hand 4 ft from the end of the flame, with the Ecoflow on you honestly couldn't get within 6 feet (which actually made the gun useless as it burned big holes in the film, but it proved a point, ended up putting it on our fork lift truck)
Like I say, just my opinion.