Hi Marc,
NZ is cool, in more ways than one, LOL.
OK, the M42 threw me, and as Robin has eluded, there is a difference to M42 Pentax as opposed T thread.
You are wanting this to supersede the Peter Tan MPCC to QHY8 adaptor then?
If so, then stick with the common thread, T thread, which as Robin says is Metric 42mmx.75 pitch.
To adjust, slightly, the length first try various T extensions, but if all else fails try Bob Luffel at Alpine Astro
http://www.alpineastro.com/eyepiece_...ed%20Couplings and there will be something in the Baader line that allows adjustment, the T2 Locking Ring for example. Maybe there is an Oz based equivalent of Bob, but I have always dealt with him.
Gary