In reply to Brendan:
You could buy proper ones....https://www.abaris.co.uk/prod/anchors/Ground-Tee-stake-Anchor.htm
But anyone with more sense than money reads the stake part of this page:-
http://www.bolt-products.com/Glue-inBoltDesign.htm
....then finds somethign suitable at the scrappy.
Scaffold tubes - easy to get hold of (cut to length with an angle grinder) but struggle with the rocky ground found on most sea cliffs. Also pretty slippery to tie onto....
Solid bar - our cliff rescue team use solid steel bar formed to a point. No idea where they came from, but are bloody heavy. They do however work well on rocky ground.
WW2 German barbed wire stakes with a T-cross section are excellent once cut to length, given a point and the side plates removed. They're either 1and5/8ths" or 2" cross section and galvanised. Google "hindernisplattenpfahl" https://forum.axishistory.com/viewtopic.php?t=178893