Grease and line with clingfilm, a loaf tin approximately 19 x 9 x 6cm; leave lots of clingfilm hanging over the sides to wrap over when ready; use about 175g of salmon to line the base and sides of the tin in a single layer, sticking it to the clingfilm.
Mix the lemon zest, horseradish, soft cheese and dill with some salt and lots of black pepper
Put ¼ of the mixture into the loaf tin and use the back of a spoon to flatten it out; pat the beetroot dry with kitchen paper and then slice into very thin rounds, add half the slices in a single layer to the terrine
Add another layer of the cheese mixture, then a layer of smoked salmon over the top; fill with another ¼ of the cheese mixture and top with the rest of the beetroot
Spoon the remaining soft cheese over the top and use the last of salmon to cover the top; fold over the clingfilm and chill for at least 3 hours or up to 24 hours