No problem, so the next question is...

Can you get a 1/2" Collet for the Makita and how well will it hold a 12mm shank (0.7 discrepancy) ?

You may get away with locking the collet nut nice and tite to take up that .7, however to save all the trial and error could you not use a cutter with a shank size you do have a collet for. Cutter diameter isn't critical here is it?, it shouldnt be because you could use a smaller cutter and make more pass's to get the finished trench width?

Lee