Will a BA interior be a straight swap into an AU as there's I'm tossing up buying a leather interior off flebay

short answer is no, I think everything from door trims to seat rails need modding, not to say it can't be done though. then the dash doesn't line up with door trims so more mods needed. so while it can be done, only a few have succeeded as it isn't a weekend job.

There are a few relevant threads around, search for ba/bf conversions.

chassis is the same so no reason it wouldnt fit. door trims are pretty much straight swap as far as i know. but all the wiring would have to be changed and not sure if you could get the BA dash loom to run of a AU engine. would be easy if you were doing a Barra conversion at the same time.

I just did NF full interior into a NC had to change the whole wiring loom from headlights to tail lights. even in the doors and floor. guessing would be the same with AU-BA but engine is completly different so not sure how you would get around that

Let me rephrase to avoid confusion
I'm just after the seats front and rear

front seats i believe will fit but im not 100%. rears i think there maybe an issue with the base and side bolsters?

I'm almost positive even the front seat rails are different. Happy to be proven wrong though.

<edit> Though not today...

http://www.fordmods.com/ford-interior-f14/ba-xr-seats-into-an-au-falcon-t80622-20.html
http://www.fordmods.com/ford-interior-f14/ba-seats-into-au-t40527.html
http://www.fordmods.com/ford-interior-f14/installing-ba-xr6-seats-into-au-xr6-ute-t66116.html