The box is the problem, not the pane
Most of what goes wrong with a sash window happens below and around the glass. Dust and grit collect in the box, in the parting bead and along the sill. Left there and dampened by condensation, that mixture holds moisture against the timber and the paint, and it is where rot and flaking start.
A sash window that is vacuumed out regularly (properly, with a brush head into the corners of the box and the runs) lasts considerably longer between redecorations than one that is only ever wiped on the inside.
Old glass is not modern glass
Pre-Victorian and much Victorian glass is thinner, softer and often slightly uneven, with the ripple that makes it worth keeping. It scratches more easily than float glass and it can be cracked by uneven pressure against a glazing bar.
So: no blades, no abrasive pads, no scouring cream, and no leaning weight into the centre of a pane. Where paint has to come off old glass, it comes off with a solvent appropriate to the paint and a plastic implement, patiently.
If a pane is crizzled or already crazed, it should be left to a glazier who works with historic glass rather than cleaned harder.
Cords, pulleys and the things that seize
Sash cords fail gradually and then suddenly. A cord that is visibly frayed at the pulley should be reported rather than used, because a sash that drops can break glass, joinery and fingers.
Painting a sash shut is the other common failure, and it is usually done with good intentions during redecoration. A window that has not been opened in years will often need to be cut free along the parting bead rather than forced.
Neither of these is a cleaning job. We note them in writing and tell you; the work belongs to a joiner.
Shutters, reveals and the stucco problem
In the stucco-fronted parts of London (Belgravia, the Notting Hill crescents, Little Venice) the frontages shed a fine, persistent dust that comes in around the sashes whenever a window is open.
It settles on sills, in the shutter boxes and along the glazing bars, and it is the reason those houses need sills, shutters and bars attended to weekly where a modern flat four streets away would be fine fortnightly.
Folding shutters need the hinge knuckles and the recess cleaned, not just the faces. The recess is where the dust actually accumulates, and it is invisible with the shutters open, which is how they are left for eleven months of the year.
Condensation
Single-glazed sashes carry condensation through most of a London winter, and in houses near water (the canal at Little Venice, the river at Chelsea) they carry more of it for longer.
The water itself is not the problem. Water left standing on a painted sill and in the reveal, night after night, is. Where a house has this pattern we treat the lower panes, sills and reveals as a weekly item through the cold months rather than a seasonal one.
Persistent black mould in a reveal is a different matter. Surface mould we treat; mould that returns within weeks is telling you about ventilation or the fabric of the building, and that is a surveyor's question.
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