Preheat the slow cooker if necessary; see the manufacturer's instructions. Lightly butter 4 individual metal moulds, each 150 ml (¼ pint), and base-line with circles of nonstick baking or greaseproof paper.
Put the crème fraîche in a bowl and gradually beat in the egg yolks. Add the lemon rind and juice and season with salt and pepper. Chop half of the basil and 75 g (3 oz) of the smoked salmon, then stir both into the crème fraîche mixture.
Pour the mixture into the prepared moulds. Stand the moulds in the slow cooker pot (there is no need to cover them with foil). Pour hot water around the moulds to come halfway up the sides, cover with the lid and cook on low for 3–3½ hours or until the moulds are set.
Remove the moulds carefully from the slow cooker using a tea towel and leave to cool at room temperature. Transfer to the refrigerator and chill for at least 4 hours or overnight.
Loosen the edges of the timbales with a knife dipped in hot water, then invert on to serving plates and remove the moulds. Smooth any rough areas with the side of the knife and remove the lining discs. Top with the remaining smoked salmon and basil leaves and garnish with lemon wedges.