Firstly again thanks for all the above.
My contributions above were all made without flats to keep it simple to start with. But Andy (and I note Robin’s comments above too) said I must use flats if I wanted to contribute to the BAA spectral Database. Having now got to grips application of flats, I recognise that this is important and it will be a standard part of my routine.
In viewing the flats, I note the ripple on the image. The ripples are parallel to the slit direction which indicates to me that the majority of the effect is in the spectroscope rather than the camera. I also see thinner striations perpendicular which I reckon are due to imperfections or dust in the slit. Anyway – these factors do need to be corrected for.
Eye-balling the ripples, I would say there is quite strong wavelength periodicity. I suggest that this is due to imperfection in the grating – maybe edge effects where one grating line meets another and mostly the same at each interface, so periodic. Manufacturing gratings with the blazing as good as it appears to be must be quite an engineering feat. When I get time, I’d like to try and model these effects.