Can an ISA be involved directly with a project for example help write the safety case?

It is not recommended and should be avoided. Any direct involvement with a safety product could compromises the independence of the ISA. The pitfalls of advice are summarised in the ISA Guidelines for MOD Projects, which states:"Suppliers and IPTs (the supplier's customer) often need in-depth independent safety advice, particularly if the ISA identifies an evidence shortfall, in what safety processes can provide the necessary additional evidence. Such advice can compromise the ISA as they then have an interest in the success of the advice. Therefore it is best to provide independent safety consultants for advice rather than the appointed ISA. Clearly, it would be the ISA's responsibility to identify areas of weakness and evidence shortfall in the safety argument."Furthermore, any advice an ISA gives should be documented so that the context of the advice is understood.  more