Hi,

I saw this film about 3 years ago (I think), but it was on television
so it could have been released much earlier than that or even be a
re-run of something really old.

I can't remember who was in it or any of the names of any of the
characters. The part of the plot that I can remember is this:-

It was set in the future, the o-zone (or similar) has been completely
depleted and no one can live much longer. Then a company sets up that
makes a force field around the earth that keeps all the bad stuff out
that used to be kept out by the o-zone layer, the company charges huge
amounts to all the governments of countries that it covers and if a
country doesnt pay up the company turns the shield off over that
country and all its people start to die. Im not sure if the company
ever did turn parts of the shield off or if it just threatened too.

In the film nothing such as aeroplanes can get through this field.
Presumably the shield was generated at a much lower height than the
o-zone layer was at. There are a group of people in the film who are
trying to turn off the shield, they are portrayed as terrorists, they
eventually break into one of the shield generators where they discover
from read outs on a console that the levels of radiation and other bad
things outside the shield are within safe limits or that the o-zone
layer has regenerated and that the shield is no longer required and
that this company has been scamming everyone for ages.

The shield gets turned off and as far as I remember thare is a happy ending,

Im not sure however if that was the main plot or if it was a sub-plot
of something entirely different.

I have been told that it was Highlander however I have read the plot
for that on the internet and it doesnt sound right. Someone else has
told me that it was Logan's Run, again the plot I read for that doesnt
sound right either.

Any help would be appreciated, it's not essential to anything, it's
just been really annoying me.

Dear timporter,

That movie is indeed Highlander - but not the original Highlander, but
rather the sequel "Highlander 2 - The Quickening".

You will see that this is correct when you read this plot summary,
provided by the Wikipedia Online Encyclopedia:
http://en.wikipedia.org/wiki/Highlander#Plot

Some excerpts from the plot description show that this is indeed the
movie you remember:

"The film's opening scene occurs on August, 1995. News broadcasts
explain that the ozone layer will be completely gone in a matter of
months and that the ozone hole has been significantly expanded and
covers most of planet Earth. In Africa millions have died due to the
effects the unfiltered sunlight has had on them. Among them is Brenda
Wyatt [...] dying with her husband Connor MacLeod by her side. She
makes the Highlander promise that he will work in solving the problem
presented. In the period following Brenda's/Sarah's death Connor
becomes the supervisor of a scientific team attempting to create an
artificial shield which will cover the planet and protect it from the
Sun's radiation. The actual head of the team is Dr. Allan Neyman
(Allan Rich). In 1999 the team succeeded in its goal and Earth gained
its artificial shield filtering the sunlight in place of the ozone
layer. Connor and Allan were proud in having apparently saved humanity
and believed they would be remembered for a thousand years. However
the shield has the side-effect of condemning the planet to a state of
constant night, and humanity can no longer see the sun, the moon, or
the stars.
The film then proceeds to the year 2024. According to a narration by
Connor, twenty-five years of darkness have had caused humanity to lose
hope and fall into a decline. The Shield has fallen under the control
of Shield Corporation and its current head David Blake (John C.
McGinley), taxing for its services in pursuit of profit. A number of
terrorists have emerged trying to take down the Shield. Among them is
Dr. Louise Marcus (Virginia Madsen), a former employee of the
Corporation. Meanwhile, Connor has discovered a negative side-effect
of his Prize. His aging process had apparently begun again since his
victory over the Kurgan. He has physically aged into a frail old man
and expects himself to eventually die of old age and natural causes.
(...)
Louise Marcus discovers that the ozone layer has been reformed and the
artificial Shield is no longer needed. The Corporation is aware of
this development but has apparently decided to hide it from the
general public in order to maintain its main source of profit. (...)
Connor succeeds in taking down the shield. Louise sees the stars for
the first time in her life."

Hope this answers your question!
Regards,
Scriptor

Excellent answer, it had all the information that I needed. The
respose was amazingly fast.