Which was the first horse to win both the English and Scottish Grand National’s in the same season?

When Red Rum passed away he was laid to rest at the winning post at Aintree racecourse in Liverpool. The home of the English Grand National.

The plural of National is Nationals. There is no need for an apostrophe, because it's not a contraction and isn't possessing anything. Plurals do not generally need apostrophes!