You must always obtain written consent from guest lecturers or speakers before you record their lecture, talk or presentation. Ideally, you should not thrust the consent form into their hands a few minutes before they start, but send it to them well in advance when you are arranging their visit.
You may want to explain to them that the University needs them to sign the form in order to be able to record them, but that they can at any stage ask for the recording to be made unavailable from our systems.
The consent forms are available from the link below:
Non Staff and Student Consent Form