Heat oven to 180C/fan 160C/gas 4. Butter 8 x 150ml ramekins. Melt the butter in a pan, then mix in the digestives, walnuts and sugar. Divide the mixture equally between the ramekins, so the base of each is covered with a layer about 1cm thick. Press down lightly on each, then cook in the oven for 10 mins until crisp and golden. Leave to cool, then turn oven down to 150C/fan 130C/gas 2.
Whisk together all remaining ingredients until you have a smooth mixture. Place the ramekins on a baking tray and pour some mixture into each one, filling to about 1cm below the rim. Carefully slide them into the oven and cook for 25-30 mins until just set. The mixture should give the faintest jiggle in the centre when gently tapped. Don’t worry if the mixture falls a little and cracks. Remove from the oven and leave to cool completely before covering with cling film and putting in the fridge. You can make to this point up to 2 days in advance.
Make the sauce by gently heating together the sugar and 50ml/2fl oz water until syrupy, about 5 mins. Then stir in half the raspberries and cook for 2 mins until they begin to burst. Strain the mixture through a sieve to remove the pips, then toss through the remaining raspberries and leave to cool.
When ready to serve, remove the cheesecakes from the fridge. Run a knife dipped in hot water around the edges. Invert, one at a time, onto a board, then turn over again onto a serving plate so the biscuit base is on the bottom. Finish by drizzling with raspberry sauce and top with strawberries.