Grease a 900g loaf tin, then line with cling film – as smooth as you can get it. Use 1 long strip of baking parchment to line the base and ends of the tin, leaving some sticking up at each end to help you pull the loaf free later.
Criss-cross another piece of parchment to line the base and longer sides. For the cream layer, soak the gelatine in cold water. Gently warm the cream and sherry in a saucepan. When the gelatine is softened, squeeze out excess water, take the cream off the heat and stir in the gelatine until dissolved.
Gently whisk in the icing sugar until smooth, then scrape into the tin and bang a couple of times to level. Chill until set – about 1 hr.
Start each following layer while the previous layer is chilling. For the custard layer, soak the gelatine in cold water. Make up the custard following pack instructions, but using the quantities we’ve given.
When the gelatine is soft, squeeze out excess water, remove the custard from the heat and stir in the gelatine until melted.
Lay cling film directly on the surface of the custard to stop a skin forming, then cool. Once room temp, scrape into the tin on top of the set cream layer, as above. Chill again until set.
Make up the raspberry jelly following pack instructions but using 300ml water. Whizz with 150g of the raspberries, then sieve.
Cool to room temp, then scatter the remaining raspberries over the set custard layer in the tin. Pour over the raspberry jelly and chill until the jelly is almost set.
Trim the brown edges from the Madeira cake, then cut it lengthways into big slices 1.5-2cm thick. Cover the jelly with sponge, like a puzzle, using as few bits as possible. Push slightly into the jelly to stick, then cover in cling film and chill until completely set, ideally overnight.
To serve, turn the tin upside-down onto your serving plate. Ease the loaf from the tin using the overhanging parchment and gently peel off the cling film, then the parchment.
Whip the final 250ml cream with the sifted icing sugar until thick enough to hold its shape. Spoon into a food bag, snip off the corner and pipe onto the top. Scatter with sprinkles, if you like.