tap washers

Hi, I have an old tap on the bath. Please see the attached image. Recently the water pressure is very low and flow is very slow. I cleaned the aerator. It is clean. The next step is to check the washer in the cartridge. But, I cannot open the top cap of the tap. How do I remove the tap cap...

I bought some replacement 5/8 tap washers from screwfix, however they are larger than 5/8ths of in Inch in diameter! What tap washers do I need for my bathrooms sink tap, the old washers are 5/8ths in overall diameter. Regards: Elliott.