Unfortunately it's going to be a process of elimination.
Is it possible that someone's fitted a new in tank pump (ITP)?
I found mine because I tend to do brim to brim fill ups.
With a bit of luck once you get the back seat out and the plastic cover removed you'll be able to see what's been going on.
Good Luck.
PS. As for a specialist, get along to a Nano meeting. There you'll meet a top flight specialist - Arctic
http://www.midlandsnanomeets.co.uk/