The turkey base freezes well so can be made ahead. Just add the mash when you're ready to finish it off.
Peas can be substituted with green beans.
You nearly got it right. As a retired chef, I first had Norfolk Pie when a child at a place in the Norfolk Broads and I have made it several times for people who doubted such existed. Your recipe is close but what is a surprise to me is that you know Norfolk Pie as few people have heard of it. Your mistake is simple. Real Norfolk Pie is the same as either Cottage Pie or Shepherd's Pie except sharp Cheddar cheese is mixed into the potato topping. That makes it a Norfolk Pie. I do not believe in rating a recipe unless I can tell the person face to face what is wrong and that is why only 1 Star along with you using turkey, which originated in the U.S.A., not England, ergo, yours is not a true English recipe. Culinary history. - 31 Jan 2016
Loved this, made a sweetpotato and potato mash for the top yum, yum, yum! Will be making this again. - 09 Nov 2015