Firstly -- FAB Fish Vase :-D
As Ron says, your vase is from the period after Michael Harris left Malta, in my opinion.
Three quick points:
The more squared off shape with near-horizontal clear 'wings' is the most obvious feature that indicates this. Earlier examples tended to be more rounded in form, as Ron says. However, in the very early days, a shape with heavier, more angular 'wings' was produced. This angular form was continued after he left, but the colours are different and the general weight and the precise shape is different. Early examples also tend to be quite small, perhaps due to the weight of the wings. I'd love to see your signed example in this shape Sue, have you got a pic you can post? It's a new one on me...are the wings angled at all? How exciting!
The precise modelling of this vase, particularly around the base, also indicates a later date to me. I've seen this sort of shape on some of the 110% later pink/blue/white examples.
The browny amethyst tinted colour also tends to be post-Harris -- in this tone. Amethyst purple and a browny purple were used in the very, very early days of the studio, but the colour is quite different, being typically darker and stronger where it does appear.
This probably just confuses matters (not my intention!), but I just thought I'd post something. Still, a cool thing I'd be happy to give shelf room to - good find!