Welcome to the Troika Cafe, and you have a great site!
Your Celtic Cross appears to be decorated by Simone Kilburn. When I am in doubt about a decorator, I consult the books I have, similar items that I might have, and the Digital Museum of Cornish Ceramics website, which has a lot of examples from Simone Kilburn.
In the case of your cross, I also look at the way the decorator wrote "Troika". Your example matches confirmed pieces by Simone.