The image itself stayed stored within the chat for less than 24 hours. I kind of just went "ok" and carried on talking, as I didn't know that it was an illegal image (as I previously said I believed that the age of sexual activity was 16 in the UK). I couldn't delete the image as it wasn't stored on my device and the only way to delete it would have been to wipe my phone, which I didn't do because as I said I didn't know that the image was illegal in any way.
As others have said this makes me guilty and I'm not arguing this, all I can say is that it was a genuine mistake and that I did not intentionally or knowingly break the law. This, however, is no defence in the eyes of the law as the defence is only valid if a person is sent the image with no prior request (true) and deletes the image immediately (false).