Morgan posted a "long length adjustable seam guide" in Machine Accessories. It sounded interesting so I followed her link. It was advertised as being suitable for Featherweight machines but would fit any machine that had the correct two holes on the needle plate. I read through the comments on the website that people had made about the accessory, and somebody said they'd bought it for their Bernina 550qe (which is similar to my 570qe) and it fitted fine - so I've ordered it.
I didn't expect the site to deliver to the UK as it's in Texas, but when I went to the checkout they stated that carriage to the UK was included in the price ($13.75) and that I would get my guide in 2-3 weeks. It shouldn't attract Customs Duty as the value is under £15. Fingers crossed.
I want to use it for a 1/4" or scant 1/4" seam, so I'll let you know how it works once I receive it. I've tried all sorts of other ways to achieve an accurate seam - some sort-of work, and some work for a while (like post-it notes that eventually lose their stickiness), so we'll see how this one does.