Put cranberries, sugar, and water in a pan, and heat gently until sugar dissolves.
Peel, core, and slice pears, and add to cranberries. Cover and cook gently for about 10 minutes, stirring occasionally, until cranberries have popped and pears are tender.
Mix arrowroot (or cornflour) to a smooth paste with a little water and add to pan (off heat). Return to heat and boil for 2 minutes, add rum and leave to cool.
Put a layer of split sponges in base of trifle dish, then a layer of blackcurrants and repeat layers. Chill well.
Spread over the custard and then the cream, and scatter over with toasted almonds.