gold telephone type bath mixer taps dont close ?

wont turn off, they go loose and when i turn off again it does not close off, it still drips both hot and cold. can i buy a new assembly for the insides, but my gold crossheads have a fine spline to them and not a course one like the new tap insides. any one help please , also my new condensing boiler keeps running because of the leaky bathtap mixer, it must think its drawing water as it is most annoying,it must be that b/tap thanyou mr foster.