Line a 240mm x 130mm loaf tin with baking paper or cling wrap, allowing it to extend over the sides for easy removal. Make up both sachets of mousse from the box as per packet instructions.
Cover the base of the tin with a layer of biscuits and top with a layer of mousse. Repeat this process until all the biscuits and mousse have been used, finishing with a layer of mousse. Cover with cling wrap and refrigerate.
Remove terrine from the tin by the lining. Slice and serve with a little whipped cream and berries, if desired.