Is the relationship between EDDC and EDBF that of a “joint body”?

No.

East Devon District Council is claiming that its relationship with the East Devon Business Forum is that of a joint body.  So what is a joint body?

According to the article on Local Government Organisation in Wikipedia, if a joint body is legally required to exist, it is known as a joint-board. However, local authorities sometimes create joint bodies voluntarily and these are known as joint-committees.  These turn out to be  arrangements between councils and other public entities such as police, fire and health authorities NOT between a local authority and a groups of businessmen, developers and landowners.

There is therefore no such thing as an official relationship between a local authority and such a group of businessmen.  The relationship is NOT that of a joint body.

What the Chief Executive seems to be saying is that he would LIKE it to be seen as a joint body, because joint bodies are in practice almost totally unaccountable to the public.  Recall that a recent joint body in this part of the world – the Devon and Somerset Fire and Rescue Authority – got into a lot of hot water (HERE)  when it voted itself an increase in the allowances of its members of 25% whilst firefighters are enduring a pay freeze that has so far lasted more than four years.