pressure rising on my worcester 9.24 combi?

the pressure on my worcester 9.24 keeps rising above the recommended pressure,i have checked the top up valve and that is not passing,i have to keep bleeding the radiators to bring the pressure down but it keeps rising,any ideas what is causing this and if i need spare parts does anybody know were i can get them as i have been told that parts are hard to get due to the age of the combi,thanks in advance for any help