I used it on the garden gate 2 years ago, I need to redo the gate because it is flaking. To be fair that's 2 years in tropical sunlight but still disappointing.
I suspect that Hammerite is a brand, and gets sold from pillar to post from one company to the next, resulting in formulation changes, probably driven by COSSH requirements on storage, volatility etc, and nothing to do with durability, longevity or protection.
I painted a large rotary clothes line 30 years ago with Hammerite and its is still perfect, but I am sure that it is not the same as the Modern product