OK just back from t'garage, and after a bit of head-scratching I figured it out. I was trying to fit the bottom two loops of the seat back
inside the rectangular brackets at the bottom, whereas it seems they are supposed to sit in front. I didn't remove any bolts when I took the seats out, but I'm guessing those are supposed to be bolted to the brackets? I was sliding them inside assuming that's what held them in place and because they do actually fit inside, apart from, er, the last inch or so! So, I still have the gaps Ade describes when the seat base is pushed into place, but not the great yawning chasms I had before, and the seat back is now flush with the parcel shelf, so I'm happy! Thanks for the advice chaps