Clean the sponge/foam in your filter with tank water(not tap water) at least once a week. Carbon should be replaced with new about every 4 weeks. However, can you give more detailed info on your setup? If you are not overstocked, perform a weekly water change of 25-50%. Though if you want to perform 50%, it is safer to perform a water change twice a week, 25% each time.

Btw, which part of UK are you?