Hi, is it possible to repair misted double glazing units, we've got a few windows that are misted but they're stained glass so swapping for a new unit is not really an option without spending shed loads of cash.

Get your wallet out I am afraid

Remove them and take them to a D/G fabricator, they'll take them apart and re-build them, might save you some of the cost. There's no way of doing it yourself, unless you have the rig to de-bond, clean spotlessly, re-bond and re-fil (with argon IIRC) after vacuuming and drying the air inside the unit.