Stuart Noble was thinking very hard :
Sorry, I didn't make myself clear. The film is between the two panes. If it
were not I could simply wipe it off (i.e. clean the windows!).

I was told that the space between the panes was originally evacuated, and
when the vacuum fails (after 8 - 10 years) the filming occurs and the unit
needs to be replaced. If this is true I would be tempted to get rid of the
double glazing and go back to normal windows that at least could be cleaned
properly.


I don't think that gap was ever a vacuum.
I've never had this happen on dg units, even on one that I accidentally
cracked. That used to steam up very slightly in the corner where the crack
was, but never got any worse.

25 years old and no such problems here. The gap is not a vacuum, but
filled to a similar or slightly less pressure to normal atmosphere.
Were it a complete vacuum, then there would be about 14 pounds per
square inch of pressure on the glass - multiplied by the area of the
glass. The seals should not fail after 8 to 10 years, they have an
indefinite lifetime, but most companies seem to provide a 10 year
guarantee - then disappear. Check the terms of your guarantee.

I have heard of one DIY solution, if you are completely stuck. It
involves drilling holes in the seal between the panels in two opposite
corners and circulating warm dry air through for some days, before
re-sealing. From memory the method used silica gell in a sealed box,
with a fish tank pump to circulate the air via small pipes in a closed
loop through the box, pump and window. Then re-seal the holes with
silicon sealer as you withdraw the pipes with the idea to leave a
slight vacuum between the panes. It is supposed to be very effective
and the slight vacuum helps pull the seals in.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk