Well you can wait until something goes wrong, which is what happened to me, pipe leaked in my bedroom while I was away! Caused about 2k worth of damage, and i consider myself lucky. I Replaced all rads and pipes in 3 bed semi for approx 3k last year. Heat from new rads is 100 times better now. Plumber at the time told me the GB pipes have a lifespan of 25 years, and in that during the replacement, the pipes crumbled in his hands in two spots in the house. My advice is do it now. No point in waiting for something to go wrong.