Thanks for the help. Unfortunately some of the other Singers that might have been better for FMQ, eg the 301, and I think also the 15-91, weren't sold in the UK, so my choice was limited. I have a FW, but obviously the size limits the quilting that can be done on it (by me, anyway - someone posted and said she'd quilted a double bed quilt on a FW, but I don't think I'll be trying that anytime soon). I bought the 201 partly because I just wanted another beautiful vintage machine, but also because my modern machine has never been good for FMQ so quilt tops are piling up. I wanted to see if I could find a solution that didn't involve another expensive modern machine.
The good news is that after I posted, I moved on to a bigger practice piece that had different batting, and managed to do some quite creditable FMQ, with accurate tension. My stitch lengths weren't very consistent, but that's because I'm still adjusting to the machine and to controlling the speed with just the foot pedal.
I was also unsure about where to set the pressure of the foot on the fabric. When I first put my darning foot onto the 201 and tried to sew a quilt sandwich, I could hardly move the fabric once I'd put the pressure foot down. So I altered the pressure, but probably went too far the other way. I've now studied how the darning foot sits on the fabric on my Janome, and tried to replicate that on the 201.
Of course I'm dealing with a combination of challenges here: getting used to the 201, as I don't have a lot of experience of vintage machines, and finding the right speed and rhythm for this machine. Consequently I was thrilled when I found that some of my stitching had come out well!
So the bobbin tension question is no longer so urgent, but I might do as you suggest and post a photo. Sorry not to have done so in the first place, but I assumed that all 201s would have very similar bobbin cases, so people would know what I meant.
Incidentally, I failed to understand the instructions in the manual for removing the bobbin case - it seemed to involve inserting a screwdriver at a particular point, to lever the case out, and as I couldn't really follow what they meant, I decided I'd better leave well alone in case I caused some damage. If anyone can give me a clearer explanation, I'd be grateful.