Thanks for the replies and links, which I will read in detail shorty.
A bit more detail (didn't want to drip feed ...) This is a very large, national organisation and I volunteer for a local section of it. As part of my work, I was asked to take some photographs to use on the local social media in support of a fund raising event. I took the pictures (using my own camera and in my own time), handed them over and they were used for the purpose stated. I have no problems with that at all.
I have now been asked to sign a form assigning copyright of "all images I take as part of my work or volunteering with " to the charity allowing them to do with them as they wish, including using them for other purposes or selling them on to third parties for profit.
I don't want this this get into a debate about copyright law if possible, but I'm querying their right to demand any such assignment.
Effectively, i have produced a product which I have passed on to them for their use for a specific purpose. It was used for that purpose. If I was a paid employee taking the pictures as part of my employment, then fine, but I feel this is a different case, so I guess what I'm trying to get to is whether I am an employee in that sense and thus subject to the constraints of being employed, or whether I am a freelancer with no such constraints.
I should add that the images have no commercial value for me, but I may wish to use them for other purposes in the future, such as exhibiting them in photography exhibitions - which I would not be able to do if I do not own the copyright. I hope that makes sense?!