Hi Matt,
The fair dealing exceptions are very context specific. There are no simple guidelines, say based on the duration of the extrract you would like to use. And it doesn't depend on what the source broadcaster says is 'fair', although clearly that may affect their attitude about whether to make a claim against you.
The main criterion is that it is necessary to use the clip because there is no other way of communicating what you wish to say in the documentary. In other words if the news item is the best evidence it is more likely to be fair dealing when the alternative, for instance just a narrator stating what the clip would have shown, is much less authoratative or informative. But you may only use the absolute minimum necessary for that purpose. Using broadcast footage just to pad out an otherwise boring bit of voiceover would be far less likely to meet the fair dealing threshold. It would be better to use stock footage for the latter purpose
And under UK law it makes no real difference what medium you are using to broadcast your documentary. Possibly if you wanted to do this under US law, then their fair use criteria which include the opportunity for a transformative use defence, might provide greater flexibility on this issue, for instance if your documentary was aimed at an entirely different audience to that envisaged by the broadcaster.
Bear in mind there are two separate categories of
fair dealing in section 30 of the Copyright Designs and Patents Act 1988, which might apply: the general one for quotation (section 30 (1ZA)) which can apply to any sort of source material such as a musical work, a written work or a video work, for example, and the slightly narrower type of fair dealing for the specific purpose of news reporting (section 30 (2)) which cannot be used to cover the use of still images. If the purpose of the documentary is criticise the broadcast itself, then you have much more leeway because you may need to show parts of the original footage in order that the viewer of your documentary understands the context in which your criticism is made*. The second use, news reporting has a long history going back beyond the nineteenth century and is generally seen as having a much broader scope. Originally it was based on the republication of newspaper reports. Because much of a news report is factual - and facts alone are not subject to copyright - other papers were able to re-use substantial parts of their rivals' reporting by just leaving out any opinion or commentary. Both categories of fair use require you to acknowledge the source of anything you use, either with an onscreen subtitle, or in the credits at the end of the documentary.
It is important that you do as much research as possible on the source material. If for instance a story was broadcast on the BBC news but the footage originally came from the Press Association or Reuters for example, then clearly this was agency footage and there would be a greater ethical reason to obtain a licence to use the same footage from PA or Reuters, than say if it was footage shot by the broadcaster themselves.
The best test is to put yourself in the shoes of the broadcaster, and see whether you would consider your use of their material was fair, if the roles were reversed, and you had obtained unique footage which they then used under the same fair dealing rules, without your permission.
* This would still not allow you to overquote from the original. For some more guidance on this see
this video by a barrister who was accused of copyright infringement by the BBC.