My neighbour has just bought a 2004 X5 3.0d and I noticed he has an aux input in his glovebox. As the satnav/radio is identical to mine, could I retrofit an aux input using Bmw parts? 2010 MY Vogue SE TDV8 3.6 Stornoway Grey- fully loaded

Yes you can

You need to source a BMW version of the BM54 tuner preferably from 2004 onwards (KW:**/04) which will have the aux input option (although this will not show up until the aux kit is connected) You then need to buy the BMW aux input kit with the black plug containing three wires which can then be removed and installed into the main wiring harness plug of your FFRR. (pins 3,4 & 10)

Take a look at the following instructions:

https://www.bavauto.com/Assets/inst_pages/ins376.pdf

You need the 502 version (Cable 'B')

Well this is how I did it many, many years ago!

Andy. 2015 L405 SDV8 Autobiography, Indus Silver with Black Pack.
SWMBO: 22MY Defender 110 HSE X Dynamic, Gondwana Stone.
Gone: 2014 Discovery 4 HSE Luxury Firenze Red
Gone: 2006MY Java Black TD6 Autobiography
Gone: 2013 Jaguar XKR Convertible, Italian Racing Red with Ivory Seats
Gone: 2010MY Stornoway Grey TDV8 Vogue SE with RSE
Gone: 2008 Disco 3 HSE, Hybrid Tv + Retro-fit RSE System & DVD
Gone: 2003 Epsom Green 4.4i Vogue, LPG, Mk4 Nav, BMW Bluetooth Phone, Hybrid Tv Tuner,

Takes me back few years - I remember doing that mod as well.

I got a BM454 from eBay and sold my BM53 for almost as much

Good thread here and loads more if you search for BM54