I raised the issue of Hammerite paint last year under the "Hammerite has changed" title. I received a couple of proper replies which explained it all, but they are not coming out here and there are some things to add. My problem was with painting front gates with the new formula and I have held off thanking the best answers until my job had weathered somewhat, The gates are OK after their first winter but the finish remains a very wishy-washy black.
The first question that should have been asked here is "Did the old tin (assuming you still have it) have the name AzkoNobel on it. If it does, then it is the modern stuff. Since you seem to have had the old tin for some time and it had lasted through several jobs, then I would say it is the old formula since the modern substance skins heavily. I make the following points which are not being made very clear:
A) If it says AzkoNobel on the tin, then it is the fairly useless modern stuff and no-one now produces the paint to the original formula.
B) If it says AzkoNobel on the tin, make no attempt to mix it with Hammerite that does not say AzkoNobel on the tin, and the thinners are not to be mixed up either as they are quite different.
C) If it says AzkoNobel on the tin, and you are painting something small or taking time over it then the stuff will start to skin around the edges of the tin, particularly in warm weather.
D) If it says AzkoNobel on the tin, then the paint will quickly skin heavily in a part used state and will go off entirely over time, even if unopened! The original paint kept for ever, forming no skin and could be thinned if it got a bit thick.
E) If the original tin did not say AzkoNobel on it and there is or ever was an AzkoNobel paint of the same name, then it will most likely not be anything like the same colour or finish.
F) I would guess the thinner for the modern mix is a bit useless, but have no knowledge. For reasons of skinning I would also guess that it does not mix to a different colour well and certainly would not keep at all.
So now you know what to look for and ask about
Dennis