Apologies - Ive done some more digging and followed further back some of the threads suggested and found the answer (or at least an answer).
If i use the string read from my excel spreadsheet as input to a FamilyType.ByName node then this outputs the Family Type. I can then use this as input to the FamilyInstance.SetType node.
Many thanks for your help - i am amazed at the responses on this forum - really useful - its a fantastic resource.