There's a pin in a google map on their site here.
https://www.arbuckles.biz/locations/downham-market/
'fraid I'm not too familiar with that part of Norfolk either. I would head for Thetford and then through Mundford on the A134. There looks like an interesting road from Stoke Ferry via West Dereham to the A10 at Denver, then North (right) to Downham Market and Arbuckle's.
Or stay on the A134 and turn left at The Foldgate Inn at Stradsett, then South (left) when you reach the A10 roundabout.