I'm in a hard water area so I clean mine every week .. I remove it and turn the hose on it, spraying the water from the rear. I then cut a lime/lemon in half and rub it on the grid, which is then replaced.
Go to B+Q buy a tub of caustic soda. Dilute this at 1 to 1 and paint on with a cheap paintbrush (I say cheap because the caustic soda will kill it after a few cleans!) and leave for 5 mins. Aftr 5 mins wash off with a hose and clean water. This cleans the bio film that buils up and makes the sieve run like new. CAUTION:- Wear rubber gloves!