Spoke to Lexus Chester, and they only fit if they sell the kit itself.
But, I got the name of the company they use for these installs, and contacted them direct.
First - some pics of the work
Microphone location

control button

While he was there, he also ran a feed for my Road Angel 2

so it all looks like this


Its all really tidy, with hardly a wire in sight (about 3 inches of cable for the RA2 is visible).
The work was done by
JA Installations
79 Ashton Lane
Cheshire
M33 5P3
The engineer was John Adams - his contact numbers are
0161 905 3108 and 07968 484 030.
I was charged £82.25, including VAT - which I thought was pretty reasonable.