hi
just had a look at xf engine bay pics and it looks like a right night mare to get the bonnet open
they look similar to a S-type, but cant be accessed from underneath, would try the basics first, get someone to lean on the bonnet, front leading edge corners while someone pulls the bonnet pull
other wise looks like the only easy way in is throught the grill, as a last resort, would have to scrifice the grill t get in, on the S-types theres, security metal covers over the locks so they cant be open through the grill, first thing i removed on my S-type, makes getting in easier if cable snaps
you might be able to hook the bonnet latches through the grill, with some long rigid wire with a small hook on the end
if your in the AA you could try them, It would not cost anything and they might of come accross it before
there really in a awkward place on the xf, pic below
hope you get it sorted
cheers
Joe