So had a look at the website where you can check your cars MOT history
https://www.gov.uk/check-mot-history today as I only just found out about it.
Interestingly I noticed that on one of the MOTs prior to my ownership (2012 MOT and I bought in 2014) there was an advisory for 'Play in steering rack inner joint(s)'. Doing some google searching of this it appears to mean the inner part of the track rod shown here on realoem
http://www.realoem.com/bmw/enUS/showparts?id=DU52-EUR-11-2007-E86-BMW-Z4_30si&diagId=32_1371 as parts 4 and 5.
Now I have a folder full of reciepts for the car right back to 2009 and non make mention of these ever being replaced (although no subsequent MOTs made mention either).
I know that the two previous owners never drove the car on the motorway as it was only a weekend A road car, TBH I dont think they pushed the car very much at all, so im wondering if this could be the problem with the steering?!
I did find a few forum posts for other BMWs (E36 & E46 mainly) and other cars such as Renaultsport Clios which claimed they had similar high speed steering wobble which was fixed by replacing the who track rod assembly.
So I guess the question is has anyone else changed these on a problemtic zed?!?!