Well... Lewis was once again vague..but from what I got from the narnia wiki is that the accident happened in 1949...... where that came from, I have no idea....
and... I think I may be reading your sequel.... o.o
Now, for a new golden age fanfic I had in mind was, that about two years before Horse, Edmund's away in the Lone Islands, Peter and Lucy are given poisoned wine meant for Susan, leaving her to care for them... and Edmund borrowed the cordial....
-ahem-
sounds like it reeks of cliche right?