You need a 900g loaf tin or terrine mould. Cut the salmon side into bars 2.5cm thick and the same length as your mould.
Place the salmon bars in half the beetroot juice to colour for around an hour. Mix the Maldon salt, caster sugar, ground black pepper and half the chopped dill together in a large bowl. Then drain the salmon bars; roll them in the salt and sugar mix to coat them with it. Put the bars of salmon into a deep baking tray , cover with cling film and leave them in the fridge to marinade for 8 hours or overnight.
The next day, line the terrine mould with cling film, and then line it with the smoked salmon, allowing the ends to fall over the sides of the mould. Very gently warm the remaining 125ml of beetroot juice and melt the leaves of gelatine into the juice. Drain the salmon bars from their marinade, wash off any excess mixture and pat them dry with kitchen roll.
Finish your terrine by pouring a little beetroot juice in the bottom of the terrine, on top of the smoked salmon. Then lie the salmon bars on top and sprinkle them with chopped dill. Spoon over a little of the beetroot juice and melted gelatine mix. Repeat this layering process as the terrine fills to the top, then fold the smoked salmon ends over the top to seal it. Compress it with a weighted board and leave it to press overnight.
Serve with fresh horseradish and watercress salad.