On Friday, 27 July 2012 20:30:25 UTC+1, John Rumm wrote:
Is there a recommended / easy way to restore a seal to these?

The sealant for new leaded glass is just chalk whiting and linseed oil - like an extremely thin putty. At the same time, a mechanical caulking of the lead flange by rubbing it down with a bone. However it's also recognised that doing this to old (brittle) lead is a bit of a fruitless task. In many cases, by the time the window has got to the stage of leaking, it's actually doing this through cracked joints in the lead. The only real solution at this point is to strip the glass apart and rebuild the panel with new lead (not actually too expensive, compared to cutting glass for a new panel).

Captain Tolley's is good stuff though, and worth a try.