Soak the gelatine leaves in a small bowl of cold water for 5 minutes, or until soft. Meanwhile, combine the double cream, milk, 50g of caster sugar and vanilla in a medium saucepan and bring to a simmer, stirring from time to time, being careful to not let it boil. Remove from the heat and allow to cool for a few minutes.
Remove the gelatine leaves from the bowl, drain and squeeze out the excess water before stirring into the cream mixture until fully dissolved.
Divide into 4 glasses and place in the fridge to set for at least 2 hours – or overnight if you like, but it would then be advisable to cover each one with cling film.
Meanwhile, to make the coulis, place the strawberries, remaining sugar and lemon juice in a stainless steel pan and slowly warm through. Once the strawberry juices run, simmer for 5-10 minutes, stirring now and again, or until quite thick and syrupy. Push through a fine sieve into a bowl and chill in the fridge until needed.
When ready to serve, carefully pour the strawberry coulis on to each pannacotta and place a few strawberry quarters on top.