Sunshine,
By far the easiest way of doing leaded glass to get a spray bottle that will spray to a fine mist
( any garden centre or DIY shed for about £1:50 ) then if you have any.. use your usuall mix of GG4 and water and just lightly mist spray each pane , use 1 cloth to clean the glass by wiping away the spray you have just applied, then buff with another dry scrim or Huck towel.
If you don't have GG4 then use water with a very small dash of your usual soap and add some household ammonia. Use in the same way as described above.
The results are great for very little extra work, charge a lot more though as you can bet the owner knows exactly
how difficult they are to clean... thats why you've got the job.
Hope this helps
Denzle