Be careful here. It depends what extra exam support you want your child to have.
The vast majority of exam concessions at GCSE level require no formal assessment at all, students are entitled to work the way that they normally do. So if he/she usually typea or dictates that is fine and needs no assessment.
If you want extra time then some aort of assessment may be needed. However, in my experience extra time is rarely useful for any student that is entitled to it. The issue is not work time, it is processing time and stress, they normally 'finish' as quickly as everyone else.
However, the option to take regular supervised breaks requires no assessment or form filling at all as is often far more useful than extra time. It can be used for a moment of respite, rest, thought or to just relieve a bit of stress. This is often far more useful.
Up to GCSE most teachers are perfectly allowed to complete the Form8 as long as they take care to follow the guidance in the JCQ handbook.
It is only after GCSEs that a new assessment should need doing, as it is often a transition period for most students and a good time to reassess needs.
Basically, the school should be able to and in fact should be completing a form8 without any additional assessment for most students that need it, and your pre-existing assessment is most likely still valid enough for any evidence required.