It's finished and I'm rather pleased with it too.
First I went all around the outer edge with the green slate, incorporating alternate red and turquoise beads every other stitch - I didn't like some of the pieces that had beads on every stitch as I felt it looked a bit too heavy.
For the top, I used rows of dc and trebles to make a flap to tuck over the top of the chair to hold it in place.
For the lower edge, I'd planned to use one or another fancy edgings from Nicky Epstein's book but each version I tried looked too fussy. I did some careful calculations to make sure I had the right number of stitches and started off with a really elaborate, dangly one which looked OK but I felt it distracted the eye from the main pattern which is not what I wanted. The next one I tried was much smaller but it was still hogging the limelight. Of course I was too eager to rip them back so forgot to take photos to illustrate my point!
Then finally I realised that less is more and just did a very simple pattern I devised without reference to a pattern - (dc, half dc, treble, half dc, dc) into one stitch, miss 1 stitch then 1dc into next 2 stitches. It gave a nice simple pointy edge which doesn't interfere with the main body of the crochet and gives it a nice balanced look.
The next thing I'm going to make using my crochet club patterns will be a mat for the top of the grand piano but in the meantime I'll be finishing off my latest knitting project (the Louisa Harding top).