Thanks for the responses. The figures I quoted in the original post were representative of ones I got from a number of different sized prints.

I started using S3d, but found I was not getting as good results as I did with Cura, but now that I have a week or so of printing under my belt I have gone back to it and realised that the original problem was almost certainly operator incompetence! Glad to hear circles are a problem for others as well (not glad as in pleased, but glad it's not just me. The good news is, the inaccuracies in the circles are fairly consistent so I just take them into account when I draw the solidworks part. Not the ultimate solution, but it works, and after all it's only a hobby!