My shortcrust pastry is always crumbly and breaks up on the rolling pin. What could be wrong?
If your shortcrust pastry is crumbling when you roll it out then it is most likely that the dough is too dry and you haven't added quite enough liquid to it. Nigella has a detailed shortcrust pastry recipe and method in How To Eat (p41) which has proved over time to be very reliable, or you could try the sweet shortcrust pastry from Nigella's Star Topped Mince Pies.
Unfortunately all flours have different absorbtion rates so may require different amounts of liquid. Once you have combined the fats and flour add most of the liquid specified in the recipe and quickly mix it in, either using a palette knife if making the pastry in a bowl, or by pulsing the food processor a few times. Then squish a small amount of the dough between your fingers and see if it holds together (make sure if using a food processor that you turn it off first). If the pastry feels dry then add the remaining liquid, mix and test again. The dough is ready when the crumbs hold together easily, but don't feel wet. You may find that you need slightly more or slightly less liquid than specified in the recipe.
It also helps to wrap the pastry in clingfilm (plastic wrap) and let it rest in the fridge for at least 30 minutes before rolling it out. If the pastry has chilled for longer then you may need to let it come back to room temperature before you roll it out, if it is too cold it will crumble and crack as you roll it.