Stretch on an angle in AutoCAD

Hi all.
Here's a problem I can't seem to find a solution to, hoping someone here can
help. I want to stretch an item I have drawn on an angle using x,y numbers.
I turn my UCS to the angle i.e. 15 degrees. I then want to stretch my item
in the x plain 30mm, so I click stretch, pick the objects I want to stretch
and then feed in amount I want to stretch 30, 0. 30mm in the x plain and 0mm
in the y plain. What happens then is it does stretch my item 30,0 but it
does it as if the UCS was still in the typical x plain horizontal to the
screen plain, yet if I do the same thing with copy or move, it works, but
stretching doesn't.
Any ideas anyone??
Ta.
Regards,
René