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I have a very large picture window with a failed double glazing panel. It mists up internally, often when the sun shines. From looking at other advice, I recon it has a failed seal. All advice seems to be just to replace the unit, but this is a big (expensive) one.

If I drilled (with a ceramic drill) into the internal glass at the top and bottom, would it allow the condensation to evaporate and clear the panel??? Presumably I could reseal the small holes after the panel clears?? Seems a lot simpler than a new panel, but why don't people do it-apart from fuelling the replacement unit market?

Because its impossible in a DIY way Captain. See our project on condensation in sealed units. Specialist equipment is needed to fit de-fogging valves.

Sealed units can be cleared of mist with ease. Remove the unit, drill holes int the edge, through duct tapr, mastic and the aluminium tube that holds the silica, so that the holes break into the 'sealed' gap. I put tree in the top edge and two in each side. I would advise against drilling the bottom in case water accumulates there. I am only two days in, but I am confident it will be ok. If after a while I need to replace the unit I have lost nothing, only the hour it took me to take the unit out, drill the holes, and put it back again. Needless to say, the trade don't want you to know this.

Sealed units cannot be repaired.
They a made in controlled conditions.
The aluminium spacer bar is filled wuth silica gel dessicant that draws the moisture out of the air in the cavity.
Misting of the unit is because the seal between the glass and spacer bar has broken down allowing more air, containing moisture, into the cavity.
The moisture condenses on the inside of the glass.
Therefore introducing holes will only make the problem worse and you will lose the insulation properties of the sealed double glazed unit.

this reply is not strictly true

Sealed units cannot be repaired.
[color=red]they can but not with in the scope of a DIYer[/color]
They a made in controlled conditions.
[color=red]the space between the glass is just trapped air no controlled conditions are used in manufacturing[/color]
The aluminium spacer bar is filled wuth silica gel dessicant that draws the moisture out of the air in the cavity.
[color=red]the spacer bar is filled with silica dessicant but is not a gel [/color]
Misting of the unit is because the seal between the glass and spacer bar has broken down allowing more air, containing moisture, into the cavity.
The moisture condenses on the inside of the glass.
[color=red]broken down unit does not mean the seals have gone but the silica has come to the end of it working life and can not absorb any more moisture the damp air gets into the cavity by solar pumping you can do a search on this[/color]
Therefore introducing holes will only make the problem worse and you will lose the insulation properties of the sealed double glazed unit.
[color=red]making holes wil not make the problem worse but will not help the situation but a small valve can be fitted to hole which help restore DGU properites[/color]

your idea is spot on. don't listen to anyone who says units carn't be repaired. what they actually mean is they carn't repair them.

thousands of units have allready been repaired.

Sealed units can be repaired bu not by DIY enthusiasts drilling holes through the seperation beads. This is at best temporary and at worst ruins the insulation value of your propoerty windows. Fitting valves in small holes is the only way to repair and 99.99% of DIY enthusiasts do not have the equipment to do this. See our project section on Condensation in Double Glazing.