Hi, I resolved this in a different way that originally intended in my opening post. I used an autocomplete field that looked up to the users api. Not a complete fail safe as it isn't constrained to only these users so if the person doing the data entry is stupid enough to type someone in who isn't in the list then the workflow doesn't work. The intention of the field is clearly to assign it to someone so the training is such that it has to be someone in the lookup list.
I can let you know more details of how I did this autocomplete field if you need it.