Well very simple : you have two paths - either buy a Leica M 90mm Macro, which does 1:2 at best with the extra googles and the whole setup is expensive or get a Visioflex, which basically turns the M8 ( or any M ) into a SLR, by adding a reflex mirror.
Now from there you either : browse around for the Visoflex mounts and lenses ( which sometimes can be a bit tricky to find ) or get an adaptor for example, to a R lens and then get a Leica R Macro lens...
Either way, you going to spend a bit of money...
A Leica M 90mm Macro costs around 1500 pounds.... goggles 300-400 ( all this used ).
Visoflex, in good condition can be had for 200-300 pounds... lens and the rest for 100-150 pounds...
I'm a big fan of Macro work ( also tele ) and the problems I found with either option is :
1 - Macro Elmar is pricey : I dont have every Leica lens that I want and to spend this amount of money I prefer to spend in a lens that I use more often ; dont get me wrong, its a tack sharp, superb lens but...
2 - Visoflex does what it says in the tin but makes the M8 or M9 look like a whole frankencamera and its just unbalanced. Granted, when doing macro work you usually have your camera on tripod but...
So I did something quite different :
Found myself a used D2H and a Nikon 105mm Macro. 300 pounds. Done !
D2H for 190 pounds, in one of my favorite shops - ApertureUK. And 110 pounds for the Macro lens. Its a nice setup, allows me to do few things that I cant with the M8 :
more precise focus control
use of more advanced flash
use of long lens
I'm just thrilled with it - great camera that can be picked up for virtually nothing these days... And to be honest, if you really love macro , just swap a D2h for a D2x, which can be found around for 700 pounds or so...
I love Leica but... macro isnt really a camp where Leicas are confortable. Yes it can be done but with lot of ifs and buts.