What information is required at the start of the transfer (old review meeting) all I have be given is a form with just a few details from my son's teachers. Also who needs to be at this meeting?
Exactly how the transfer process runs does vary between local authorities, but it seems to be pretty much standard practice for schools to be asked to complete a form that covers both the usual statement review requirements (child's progress, is the provision still correct, have the child's SEN changed etc.) and starts to think about what needs to be in the EHC Plan (more information about your and your child's views, deciding upon outcomes to be worked towards etc.).
Some schools are filling out this form as far as they can before the review meeting and sharing it with parents and then finishing off filling it in at the review meeting. Others are just gathering teachers' views on progress, provision etc. which will be supporting information to the form when it is completed, sending those to parents in advance of the review meeting, and then filling out the form from the meeting. It sounds to me as though you might be in the latter case here. If so, I think it is worth asking the school or the local authority for the form that will be completed by the school following the review meeting so you know what will be covered, and can be prepared to put your views over in the meeting.
At a review meeting, there must be a representative from the local authority, relevant school staff such as the SENCo, class teacher, TA etc., and external professionals who are working with your child e.g. speech and language therapist, occupational therapist, advisory teachers such as literacy or ASD specialists, educational psychologist. All the external professionals may not be able to attend the meeting, but anyone who is currently working with your child should provide a report that you should have at least two weeks before the meeting so that their advice can be discussed at the meeting, even if they are not there.
It is worth remembering that a transfer to an EHC Plan involves an EHC assessment (very like the statutory assessment that was done for the first statement), and so all relevant professionals should be giving written advice, and if they have not done so, they should be asked for it by the local authority unless you, the local authority and the professional in question agree that an already existing report is suitably up to date.