Roll the pastry out to a 5 mm thickness (as thin as you can for a delicate crust). Use to line a 23cm shallow loosebottomed flan tin and leave the pastry hanging over the sides. Prick the base and chill or freeze for at least 30 minutes.
In a large bowl, mix the treacle with the golden syrup, lime zest and juice, and rum. Stir in the breadcrumbs and coconut.
Put the flan tin on a baking sheet.
Spread the syrupy filling over the pastry base.
Bake for 15 minutes, then turn the oven down to 160C/gas 3/fan 140C (to stop the mixture bubbling over) and bake for another 15 minutes, until the filling is just set and the pastry starting to brown at the edges.
Meanwhile, whisk the rum, cream and icing sugar together until the mixture stands in soft peaks and the whisk leaves a trail.
Transfer to a bowl, cover and chill until needed. Cool the tart slightly before trimming the pastry edges with a sharp knife.
