Gavin, You maybe able to buy a simple adapter (reduction sleeve) from a local shop that actually sells other DeWalt machinery: a cylindrical tube with a single cut in the longitudinal direction. You probably do not need to order from the US.
This (cylindrical tube) adapter, which you insert into the 8mm collet, decreases the inner size of the collet from 8mm to say 6mm to fit the respective drill. It will cost about 12 euro.
Also see “Continuing the discussion from [Spindle and collect size for X-carve (DeWALT 26200)”] (Spindle and collect size for X-carve (DeWALT 26200) - #9 by Angelos):