Roll out the other 1/2 of dough to a large rectangle about 1/2cm thick, and use a ruler as a template to cut out enough 30cm- and 20cm-long strips to cover the whole pie. Keep re rolling the trimmings to cut more. Draw a template of the tin on baking paper, then lay enough 30cm strips side by side to cover the space. Fold alternating strips back on themselves. Lay a 20cm strip across in the space just below the folds, then fold the halved longer strips back down – the shorter strip should be latticed into the middle. Using each newest strip as your guide, fold alternating long strips back over themselves, giving space to lay a new short strip. When the strips are unfolded the pastry should be neatly latticed together. Work up to each end of the template, until the whole space is latticed. Trim the edges neatly so the pastry top will fit snugly inside the tin. Freeze the lattice top for 20 minutes.