OK. Managed to get further with this by using an SQL table as the data source. It seems much more stable.
I've used:
1) key field is a combination of several fields on the source data to make a unique field
2) a default value as a Title attribute
3) an email lookup for the raise user that maps direct to the email address field and is set as the Target Key Attribute for the Raise User Object
4) a couple of other default attributes have been set to guide the process into the Subscribed status
5) a new bit of Process to skip out a lot of process if the Request is an imported value
6) Import to the Request object
All working pretty well now
Thanks for your help as always
Jason