Nice start Adrian.
Looks like there is either a fair bit of curvature in that lense or your alignment is off. For such a short shot though your alignment would have to be way off to produce those trails, so I'd go with field curvature.
How many shots do you have? What programs do you have? If you have registax and plenty of ram (recommend at least 512, preferably 1 gig) then you can use registax to stack them. Other wise track down IRIS (free) and download
Jim Solomons Astrophotography Cookbook