Just an update on the original question,
After many weeks of haggling between builders, roof fitters, glass installers non who wanted to take responsabality , because of the £12,000 it would cost to relplace the glass a solution has been found
The roofers admitted in the end it was their fault for not pattenising the lead, and after numerous attempts at cleaning it them selves with various solutions recommended to them, ie. white spirit, white vinegar, meths lemon juice etc they admited defeat, although a little improvment was made after labourious rubbing of just one small area. they were going to put it through their insurance was the only solution. bearing in mind nothing abrasive could be used.
I stepped in with quote to bring back the glass to pritine condition" after consultation with Pilkingtons tech dept and anaylasis of the marking" and after I had tested an area with their recomendations.
The price for cleaning 20 panes of glass £750 , they bit my hand off .
Think I may just take up window cleaning
I will take some pics tommorow to show what the problem was.
geoff