Watching Big Brother 5 recently has reminded me of a film I remember
seeing snippets of when I was very young. I'm guessing I saw it when I
was four or five, so that would put it in the early 80s.

The film was about a group of people locked in a house on an island as
part of some kind of scientific experiment. They were told they could
not leave the house because of noxious gases outside. As time went on
they all began to go crazy (and murderous?) and desperate to leave.

There was a bit where a helicopter arrived (to rescue them they
thought), but only dropped more supplies before leaving them again.

At some point they realise that the gas outside is a myth when they
discover a weasel running about quite happily without choking to
death.

So the big question is, what on earth was the film?

Could it be this movie, Terminal Island, starring a young Tom Selleck?
http://www.jabootu.com/terminalisland.htm

I don't think so.

I don't remember the people being criminals. I think they had
volunteered to take part in an experiment, either that or they
themselves were doing scientific research. I don't think the island
was actually a prizon.

I don't suppose it could be "Operation Dead End (1986)"?
It sounds a little bit like it but the people are in a shelter not a house.
Internet movie database (www.imdb.com) says:

"In order to find out what will happen to people in a concrete shelter
in a post nuclear situation some scientists are looking for three
volunteers to stay isolated on a small island..."