Cut each of the flat sponges in half (exactly down the centre) to make two long strips. Unroll the rolled up sponge and do the same. Trim any woggly edges off the sides but try and keep all the strips exactly the same size. Lay one of the strips that had been rolled up on a piece of parchment and spread a thin layer of frosting all over it. Lay the other rolled piece on top but 2cm inwards from the short edge, roll them up, folding the bottom piece up and over the top one to get the roll started. When you have a roll, lay another strip down, frost it and join it edge-to-edge onto the roll where the outside strip ends. Repeat until you have used all the strips, then upend the roll onto a cake board and make sure the top is flat, trim it if you need to. Warm the frosting a little until it is shiny and soft enough to spread easily, then frost the outside of the cake.