Preheat the oven to 180C/350F/Gas 4. First make the pastry. Cream the lactose free butter and sugar together by hand or in a food mixer (no need to over cream).
Add the eggs and beat for several minutes. Reduce speed and mix in the gluten free flour. Turn out onto a piece of floured greaseproof paper.
To make the tart, roll out the pastry 3mm/1/8in thick approximately and use about 2/3rds of it to line a 21.5cm/8¾in square x 2.5cm/1in deep with fluted edge tin.
Lay the strawberries and rhubarb in the pastry base and sprinkle with the sugar.
Cover with a lid of pastry, seal edges, decorate with pastry leaves, brush with the egg wash and bake in the preheated oven until the strawberries and rhubarb is tender, approximately 45 minutes to 1 hour (but do keep checking some ovens may vary cooking time).
When cooked, cut into squares, sprinkle lightly with caster sugar and serve with softly whipped lactose free cream and Barbados sugar.