s20

Hi all its time for a new rear tyre after 3500 miles. Not sure what way to go, should i buy just a rear Bridgestone s21 as i still got plenty of life in the front s20 is it ok to mix s20 / s21? Or stop being a tight arse and buy a set of PR4s or somthing else. I am commuting through London...

Has anyone tried the S20 Evo tyres yet? They are supposed to have the same performance as the S20 but with improved wear.