Hi,
We have the same car seat and same car and yep the same problem. Did manage to get it strapped in though I left it un-buckled and put the belt round the back diagonally then fed the lap part across and managed with strength to put the buckle together. That was the only way. you will know yourself if you put the lap across then try and put it round the back it won't fit.
you could ring kwik fit safety belts on:02082060101 where you can take the car there (middlesex i think) they will remove the belt and extend it and refit for £116!!! or remove it yourself and post it there and will cost you £50 odds!! Ford reckon there is only one length on the shelf. Apparantly they don't recommend the extension pieces you can buy as the buckle against the seat could come undone! maybe better with new seat!
good luck think we are just gonna struggle with it being tiight for now!
you probably have it sorted by now just seen your note as struggling with ours.
Kev.