Well I managed to get a telephone number out of the automated chat assistant, by repeatedly saying I didn't want to chat to an assistant online.
What a difference, the lady on the telephone was much more helpful, I explained they had sent the wrong case and I just wanted it exchanged for the right one, so while on the phone she sent me an email with a link to a customer portal, which she then talked me through (just a case of selecting why you are sending the item back) this opened up a returns ticket, couple of hours later I got an email with a returns label.
She also informed me that they had put in a priority shipment for the right case. The only slight concern is while she was talking me through the exchange on their customer portal interweb it says you want to exchange product A for Product B and product B was showing up as the case I'm sending back, I pointed this out and she explained that apparently there is a glitch with the Kobo stock system, the clear case for the Kobo Clara is showing up as the sleepcover for the Libra (which I guess is why the wrong one was sent in the first place) I will have to wait and see, hopefully I don't end up with them sending me the exact same (wrong) case again
The only other slight concern is I've had no confirmation they are actually sending the right case, only her word, but I'm sure they will.
Oh and she gave me a 20% off my next Kobo order as an apology