After all my drama in locating a cover, I ended up picking up a Proporta cover at Waterstones for my 350. You have to ASK to see covers at my Waterstones - nothing is displayed as (apparently) the covers are 'targets of opportunity' for theives.
As a plus point, the black Proporta cover is £19.99 versus the £29 for the Sony cover.
The Proporta cover is the same size as the standard Sony cover, but it is somewhat thicker and definitely stiffer. I feel it does a much better job of protecting the screen than the Sony cover will. The inside is covered in a soft short pile velveteen. This has proved useful as I have a small piece of microfibre cloth for cleaning the screen, and it clings quite nicely to the inside of the cover (I keep it behind the reader).
The spine and clips are heavier and more sturdy than the one in the Sony case. On the downside, it's placed in the spine of the cover, rather than on the back cover. I've done some DIY and relocated the spine/clips to the back cover, and I'm much happier with it there. The reader doesn't move every time I open the cover. I'm also confident that it's going to stay where I've glued it.
Again, it doesn't have the magnets to keep it closed like the sony cover (though those magnets don't seem to work as well on the back of the reader which I found frustrating). The cover folds back easily for reading, if you like to hold your reader one handed.
All in all, however, I'm happy enough with it until something better comes along. I'll see about posting some pics when I find my camera.