Wow. Just ... wow! Thank you so much for all your beautiful comments on my last post. It was only when the questions starting coming through about the quilting that I realised I had left out one very important bit of information.
How I did it.
I followed a great tutorial over at Blooming Poppies and I recommend you go over there now and check it out.
I used a clear jar to trace the centre circle but in hindsight that jar was a little small, and I should have gone for something bigger (like the food processor bowl recommended in the tutorial), and then worked my way in at the end of the quilting. Be prepared to quilt that centre circle a couple of times - it's awkward! I unpicked it once and did it again until I was happier with it.
I used the guide on my walking foot and was happy for this quilt to have even circles. For the next quilt though I am hoping to mix up the widths a little.
For the commenter who wondered whether I thought they could quilt a very large quilt this way, I say why not? I wouldn't, but I don't quilt anything on my machine bigger than 60 inches. That's a personal preference based on my physical limitations and my unwillingness to spend hundreds of dollars at the physiotherapist. As long as you are prepared to go slow and keep your needle down when you turn the quilt under the machine, anything is possible!
Hope that clears up some of the questions - if it doesn't please ask in the comments and I will endeavour to answer them there.
(Forgive any tardiness though - I have a cracking case of gastro at the moment and I'll be offline for a bit while I recover.)