You should pull up and remove weeds by hand as and when they appear. If it is a regular problem, you might need to reseal the paving. In extreme cases, the sub layer itself may need to be re-laid. You can use weed killer, however some paving can be discoloured by stronger weed killers.
Crymych
Crymych is a town of around 800 citizens and also a neighborhood (population 1,739) in the northeast of Pembrokeshire, Wales. It is positioned around 800 feet (240 m) over water level at the eastern end of the Preseli Mountains. It stands astride the old Tenby to Cardigan turnpike road, now the A478. The town developed around the previous Crymmych Arms train station on the now-closed Whitland to Cardigan Railway, nicknamed Cardi Bach (Little Cardi). Crymych, which is twinned with Plomelin in Brittany, has a chosen area council. The town has given its name to a selecting ward of Pembrokeshire that includes the villages of Crymych itself and Eglwyswrw.