As pointed out earlier, the Advertising Standards Authority has already ruled against the claims of this product and the seller (Sussex Promotions) has been told not to repeat them:

"In the absence of evidence demonstrating that the Fuel Diamond could prevent stale fuel or neutralise ethanol, and therefore prevent associated damage to vehicles, we concluded that the claims in the ads had not been substantiated and were misleading.

[The ads] must not appear again in the forms complained of. We told Sussex Promotions t/a Fuel Diamond not to make claims that the Fuel Diamond could prevent fuel/petrol from going stale or neutralise ethanol and therefore prevent damage to vehicles, or any similar claims, unless they held adequate substantiation to support them."

www.asa.org.uk/rulings/sussex-promotions 17-August-2022

The recent marketing seems to have been watered down (pun intended) but I see they've shipped over 37 thousand of these things through their eBay store alone so I guess it's lucrative enough for them to disregard the ruling.

2011 Morgan 4/4
1932 Austin 7 Chummy