My car is on 60k and I'm about to replace all the bushes, wishbone bushes are definitely past their best, the back end seems ok but I'm certain I'll notice an improvement. You've spent a decent amount on your Nitrons so it has to be worth the relatively small investment to be sure you are getting the best out of the suspension. £96 for a complete set of bushes from Caterham. They are easy enough to replace with either a proper bush tool or a press of some sort although it can also be done with a combination of old sockets, a vice, threaded bar etc. Worth also getting a geometry/flat floor set up after everything has been replaced too.