Thanks for the comments guys
renormalised: there are limits on the resolution achievable out of this simple setup. The addition of a slit in the light path, increasing the lines/mm in the grating, increasing the distance from grating to ccd will give some improvement but you reach the limit pretty quickly. On Christian Buil's website,
http://astrosurf.com/buil/index.htm there are some mathematical proofs on how accurate you can get with a star analyser type filter.
Long exposure shots were beyond me, so I went for something that seemed, at the time, a bit easier.
Now I know different...
Mark