is it original leaded or replacement leaded?
i have a really old manor house on my round that has a mix of both, the replacement panels with upvc frames are absolutely fine, however, the originals, which are VERY old, need to be treated with tlc. Some can be wfp with very low flow, and some flex so much that they set the house alarm off, so a mist of pure water and a gentle polish with a microfibre works really well.