Hi Chris,
I feel for you and the others on here who've ordered it. Must be very annoying if that's what you really wanted.
I'll try to put a positive spin on it though! The 60/40 split seats and cargo box option could potentially work instead (we have both on our Panda) as they create a flat payload, plus you could still have one (or two skinny!) passengers in the back as well if you like.
Also, you can just take the cargo box out whenever you like to revert to the standard boot. I love having the flexibility to do whatever I want in our car; we also have the folding passenger table which, along with the cargo box and folded back seat, means I have got over 2 metres worth of flat payload to fit in things like ladders, single mattresses etc without having to break a sweat
I fully understand why you want the sliding seat functionality, but I hope you guys aren't too disheartened. What I'm trying to say is that you should be able to find a similar level of flexibility with other options.
Good luck anyway!