Third time lucky - it fitted! By now, however, I had lengths of different colours, all joined together but not matching the ends of rows. I decided to use it as is - allowing the colour change where it fell.
I then joined the front and back - using single crochet through both pieces on the inside.
Yesterday - my settlement day on both the sale of my house and the purchase of my apartment - I dashed to Spotlight and bought a cushion insert. While waiting on the call from the conveyancer I inserted it and crocheted up the last side. I considered lining the cover but didn't in the end. I didn't think the show- through was sufficiently noticeable to warrant lining - and would make washing more difficult.
So here is the finished item - now in my daughter's family extension.