So after reading all the DoT articles I still couldn't find anything that gave the definition of an anchor. So I've gone with 2 danforth anchors end to end with a metre of chain between and then 100kg of steel plates on a 10kg pipe where I can slide each plate on and off as the third heavy anchor, followed by about 5m of chain. It's only an 18ft in 1.5m of water and about 20m offshore, so three anchors should hold it.
If needed I could have the whole lot up and out the water and into the boat in about 5mins, you can't do that with a mooring so therefore it must be an anchor. I think the cleats will break before this anchor moves so no need to go any bigger

