Thats a nice looking bench!
I have two cast iron bench ends that were left in the garden by the previous occupiers of our house I was planning to add slats to to make a bench. Can I ask what the dimensions of you slats are, and whether you put a mid support under the seat as you did on the back? I presume you did! With the curved cast iron ends I have I think I am going to have to put a metal strap to support the seat slats but it would be handy to know the dimensions of yours and how solid they are.
Cheers,
Steve.