Ok, I did some more digging this morning and found this.
https://www.carfeature.de/en/hifi-na...d-bypass-cable
Have been informed by the supplier and the UK Gladen distributor that it is the correct part. Unfortunately the UK distributor doesn't sell them individually, just as part of the audio upgrade kits (which look pretty good I might add). So I've just ordered one from the link above coming in at about £50 delivered.
Much better than £110!