.....of mounting my rack mount pc inside rack unit which is angled slightly backwards.
 
An angled floor rack is slightly more ergonomic than a vertical one, but it's potentially the only place I can put the computer, so, any downsides that anyone can think of?
 
 
Cheers,
 
Jonesey

I don't think the slight angle will be an issue.

Shouldn't be a problem. You can run a PC at pretty much any angle you like, including on it's side and upside down with the only things affected being possibly the optical (CD/DVD) drive and cooling that relies on convection (hot air rising) rather than fans plus the airflow generally- you don't want to trap hot air in the case.

I agree with tlw, for the most part.   Ventilation is the priority.  I've even seen a CD/DVD drive mounted upside-down.  It worked fine, but it was a real pain to get the disk into the drive.  Even at the slight angle that you're considering, it can be a little awkward.  Make sure not to put the PC at the very top where the rack's frame might get in the way of CD/DVD disk handling.  
 
I'd also suggest using an empty space or a blank above and below the PC for ventilation.  
 
Good Luck.  

Good thoughts, and leaving an extra rack space is no problem.
 
Thanks guys.

With a Rack, heat-buildup is the biggest concern.
Leave rack space above/below the DAW... and don't put other hot gear (power-amps, etc) next to the DAW.