I assume this documentary was recorded on video (tape or solid state) and was not broadcast live without it having been recorded (highly unlikely these days). In which case the law says it is "a film". However based on what you have told us, it is difficult to provide specific advice on where you stand.
According to Section 9 of the Copyright Designs and Patents Act1988, the authors of a film are:
9 Authorship of work.
(1) In this Part “author”, in relation to a work, means the person who creates it.
(2) That person shall be taken to be—
- (aa) in the case of a sound recording, the producer;
(ab) in the case of a film, the producer and the principal director;
On that basis, it could be that your contribution on the production side was sufficient to qualify you as a producer (described as 'the person by whom the arrangements necessary for the making of the sound recording or film are undertaken'), and thus you would be an author of the film as outlined above. But it is equally possible that the charity could be the producer if they provided all the financial and planning resources to make the film, and you would only rank as an assistant producer. Next, as you appear to have been an employee of the charity at the time, even if you were considered to be an author, the charity would be the owner of the copyright. This because an employer automatically gets ownership of the copyright in a work made by an employee in the course of his or her employment.
However if you possibly were the producer, but not an employee or your work duties did not involve making films, then it is possible that the charity are not the exclusive owners of copyright. This would give you considerable bargaining power since the film could not be shown without your permission as a joint owner of copyright.
However that would be the only grounds on which you could insist on a credit. A producer has no moral right in law to be credited, only the director of a film.
I don't think, based on what you have told us, that your contribution towards the concept or the provision of the title would be sufficient to amount to a claim to authorship of the script (a literary work).
If you had either a written contract of employment or an agreement relating to your duties in respect of the documentary, you should check to see if there is any mention of the ownership of intellectual property rights, and if so whether it clarifies any of the points raised here. On the wider issue of whether you are entitled to any remuneration for your work, that will entirely depend on the contract of employment or any other agreement (either verbal or written) which was made before you started work.