It's a perennial problem and I'm not sure that anyone has really solved it. There's been a bit of a discussion on the topic here:
http://periodpropertyshop.co.uk/phpBB2/viewtopic.php?t=7796&sid=53d3dec659e7358105ad32398d31bf5e

Tony, they have already been 'Ventrolla'd', so the S/G is the icing on the cake.

Nick

try STORM WINDOWS....but alternatively it is not difficult to make you own frameless secondary glazing all you need to do is:1. make a flush i.e in same plane surround to the frame for the new glass to sit against, this can be in softwood painted/stained as the main window. There needs to be a cill (support) for the glass to sit on at the base and a head (to fix the restraint channel to) at the top. 2. buy some double sided steel magnetic/adhesive tape (comes on a roll as long as you like). 3. size glass up to fit existing openings (obviously you need to meet british standard for safety glass at low level and in hazardous positions: any glazing firm will have a pamphlet on this), and the size needs to be something that can be carefully handled (to achieve this it may be necessary to break down the size of the existing opening with intermediate supports (usually transomes) again in cheap softwood. 4. buy rubber or plastic glazing gasket to run along the lowest edge of the glass i.e at the cill, stick the magnetic tape to the jambs and to the jamb edges of the glass (clean it well), and at the top screw fix an aluminium channel to the head of the frame (this has to be wide enough for the glass and deep enough to push the glass up in to to restrain it safely. 5. make a little tool out of aluminium or zinc to slide under the base of the glass at the cill to release the pane when you want to clean (once a year is enough).

much cheaper than secondary systems with lots of al.framing.

remember conservation legislation allows incremental changes of this sort especially since it is eminently reversible and minimal.
any questions phone 01424 752311

The windows in our house are 16 pane sash made from redwood in 1857. In the time we've lived here I've done major repairs to them on two occasions. I realised that most of the damage was being caused by condensation on the panes collecting on the glazing bars and creating ideal rotting conditions. The windows were not designed for a warm relatively draught free house.
Fitting secondary glazing solved this problem. The windows are now dry. It's also a lot more comfortable without the draughts and cold glass.
I made a casement frame of 25mm softwood with two hinged sashes of 200mm softwood and sealed it with thin foam draught seal. Glazed with 4mm glass. I screwed the frame to the sash window frame. In the summer they are open and fit within the window reveals.
Matt black paint on the outer side makes them almost invisible from outside the house.