What to do about the windows?

The house has wonderful sash windows.  When we bought it we kind of assumed they were rotten to the core and would just need compete replacements.  This had some advantages in a way,  we just planned to ignore them until next year and then bite the bullet and get some modern (wood, double glazed, still sash) ones installed at some, no doubt fiendish, cost.

Two things have happened since, one is that I've learn a lot more about sash windows,  and have realised the windows aren't in too bad condition.  The other is that we had a cold snap the other week...

Cue streaming condensation in the bedroom and the front room (though that's got drying plaster which might be the culprit) every night.  Apart from being miserable in itself its also very likely to lead to the return of the dreaded mould.  So something needs to be done.

After very exhaustive searching it seems we have 3 options available:

1. Put in slimlite style double glazing pains into the existing sashes
2. Throw away the sashes, keep the boxes and put in new double glazed sashes
3. Have some sort of horrible, sliding, internal secondary glazing.

They're all horrible (I'll expand on this later), so I've decided to make up a fourth option...