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500mm High 800mm Length T22 Style 2000 type Double Panel Radiator With Double Convector

Also the quality of the parts matters. The replaceable parts such as flappers and washers/seals generally will last at least 4 to 5 years on "average." If you use a bowl cleaner some flappers won't even last one year.
If your flapper has black "goo" on it then it is either due to age or quality of your water and if it feels soft then it's best to change it.