You might try talking to this guy:

http://www.artwithglass.co.uk

I got a quote for a repair job a while ago but just haven't got around to doing anything about it yet so I have not used them and don't know how good they are. But when I asked at The London Door Company if they did stained glass repairs in doors they immediately suggested Art with Glass. I think one of my neighbours used them to do a repair and add secondary glazing after a break-in so they will be able to advise you. Not a cheap solution, but possibly a sound one.