I think they were standard then became an option and now i'm not sure, if they aren't listed then i'd say they aren't fitted
According to this, only the XL has sliding rear seats (and even then not in all trims), see here:
http://media.seat.co.uk/pdf/April_Altea_Spec11.pdf
My my2010 had sliding rear seats, but I've never used that function because I use the false floor and don't want that gap opening up..
I'll have some for sale in a short while. I'm selling the Altea and will be putting the standard ones back in before she goes. If you're interested. £100 I paid for them plus postage so you can have 'em for that I reckon.
Doesn't your false floor have a flap sticking up at the back, where it meets the backs of the seats? Mine does and it fills the gap when the seats go forward.