Bob:
Be sure the shift linkage is pushing the transmission arm
fully into Park. If it's a little loose or out of adjustment, it won't push the pawl into position.
That said, I did have a friend with a Jag sedan with the same trans and the pawl
was broken. Not sure how he fixed it.
Otherwise, nice to see another Brit car in NJ. We live in Morris County but we're in the Somerville area at least once a week, so I recognize the name of your town.
Will you be at the Clinton or BritFest shows?
On a related note, I saw this '58 Morris Isis sedan out on Rt. 31 sort of near Flemington a few years ago. It was in a used car place. Gone now.