Roughly tear up the bread for the filling and pulse in the food processor until it becomes fine crumbs. Tip it all out into a bowl and set to one side until needed.
Then wipe out any leftover breadcrumbs and add in the flour, cold butter and salt. Blitz briefly until it becomes like fine crumbs. Pour in 4 tbsp of cold water and blitz again briefly until it starts to clump together to form a dough.
Tip the dough out onto a large piece of cling film and bring together to form a ball. Flatten into a disc, wrap up in the cling film then pop into the fridge to chill for 20 minutes.
Meanwhile, preheat the oven to 395 degrees F (or 375 degrees, if using a convection oven) and place a heavy baking sheet in to heat up.
When the pastry is chilled, unfold the cling film and place another large piece of cling film over the top. Roll it out as thinly as you can ensuring that it's bigger than the base and sides of your tin. Then peel off one layer of cling film and carefully lay it, pastry side down/cling film side up, over the tin. Gently ease it down into the tin, pressing it lightly into the fluted edges as you go. Try not to let any air get trapped underneath.
Peel off that layer of cling film and roll a rolling pin over the top to 'cut off' any excess. (Peel that all away and put to one side for a minute.) Now prick the base all over with a fork.
If you want to, you can roll out remaining/leftover pastry thinly again and cut out a few pretty hearts, butterflies, flowers, etc to decorate the top. (This is completely optional.)
Pop your tin (and decorative shapes) into the fridge to stay chilled while you make the filling.
Pour the golden syrup into a medium saucepan and heat gently for 2-3 minutes over a low/medium heat. You want it warm and runny - DO NOT BOIL. Then add in the breadcrumbs, lemon juice & lemon zest and give it a good stir.
Take your prepared tin out the fridge and pour the filling straight in. Smooth it out with the back of a spoon. If you've made some decorations, arrange them however you like on the top. You may egg wash the decorations and the edge of the pastry, if you’d like, but it's entirely optional.
Pop your treacle tart into the oven on the preheated baking tray and bake for 10 minutes. Then reduce the temperature to 355 degrees F (or 340, if using convection) and bake for a further 30-35 minutes until a deep golden brown all over.
Allow to cool completely in the tin before removing. You may eat warm or cold and store in an airtight container at room temperature (preferably metal), consuming within 3-4 days.