In order to improve the longevity of your traditional leaded light windows, it is important that you inspect them for problems from time to time and ensure that you clean them regularly.
You can expect a little movement in leaded lights but any noticeable bows and distortion may result in panes of glass (the quarries) from breaking. Check that there are no breakages in the lead cames as that may result in draughts and will mean that the window is no longer providing a weather-tight seal. Ensure that the window frames are in good order. Rotten wooden frames or stone frames which may have slipped slightly due to soil movement can create serious issues for the whole window.
Gently clean leaded lights on the inside and outside with de-ionised water and small wads of cotton wool. Do not scrub or use abrasive cleaning products as old glass is likely to be very delicate and prone to scratches.