Hi John,
The chap from Hills Alive luggage came out, measured up and said he would get back to me. Never heard from him again!!!
I haven't seen my car since November as it's been away at a dealership and the factory. Out of sight out of mind!!
I'm of the opinion mind that due to the odd shape of the S5 boot, custom luggage may actually be more of a compromise than without. I went away last October and my wife and I used two sports bags in the middle and filled the outer edges with smaller bags/boots. Perfect.