Hay shazza, are those black strips on the XT photo, just from the chamfered edges?

For that striped thread in your block, you could try Heli-Coils. I have used then before with great success, but only in steel and ali.

I have google Heli-Coil to see if there is a Heli-Coil set just for plastic or if the standard one will work, and found this http://www.helicoil.in/
At the bottom of the web page there is an article about Dodge threaded plastic inserts that are made out of brass.
I have not had the chance to play with these, yet!, but looks good, see for yourself, hope it helps.