Roughly chop the strawberries and put in a small pan with the water, orange juice and sugar. Bring to a simmer, stirring now and then. Simmer for a couple of minutes and mash the strawberries to a pulp. Set aside to cool then strain the mixture. You can make this ahead and store in a cool place or the fridge, sealed.
To make up tiramisu (at least 4 hours before needed/night before)
In a medium-large bowl, whip the cream until it forms firm peaks.
In a separate bowl, beat together the mascarpone and sugar so the mixture is smooth and glossy. Fold in the cream so everything is well combined but try not to loose all the air.
Put the strawberry syrup in a small bowl/dish that is wide enough to fit a ladyfinger cookie in it, but not much bigger. Dip one ladyfinger cookie at a time into the strawberry liquid. Turn it over so it is coated but don't leave to soak. Put the cookie in a dish that's big enough to hold a layer of 6 cookies, but not much bigger, and around 2in/5cm deep. Repeat with more ladyfinger cookies to form a layer over the bottom of the dish. Drizzle over a small amount more of the syrup if it seems they didn't absorb that much.
Put half of the mascarpone mixture over the layer of ladyfinger cookies. It's best to dot it on then spread to join together as otherwise you may break the cookies or end up with the syrup part mixed through. Don't worry if the mixture is a bit thin, it will firm up when it chills.
Slice the strawberries relatively thinly and lay a layer over the mascarpone mixture, trying to have it well covered. You can overlap them if you like. Dip the remaining cookies in the strawberry syrup and lay over the strawberries and top with the rest of the mascarpone mixture. Cover and chill for at least 4 hours, or overnight.
Decorate with a few slices of strawberry on top and serve.