Thank you all very much for your comments - they are appreciated.
Perhaps I can answer the questions from Jim, ok wx guy 24 and Stargazer with this brief process description:
I stacked all the subs (150, 180 & 210 sec) at once in ImagesPlus 5.0 (along with the calibration frames of course). After stacking while still in IP I did a DDP stretch which included the "nth Root Xn" found in the HDR Pre Stretch Scale at the top of the DDP box. This is a fascinating tool as it enables you to aggressively stretch the fainter parts while not blowing out the stars. That is how I got the faint dust started. Following that, still in IP, I did some smoothing and star/halo reduction. At this point I transferred the image to Photoshop.
From here on I basically used our friend Scott's DSLR-LLRGB processing workflow that many of you are probably aware of:http://www.astronome...gb_tutorial.php
And thanks again Scott for taking the time and effort to make this tutorial - great job.
When I finished processing the image I still had to combine the shorter (15 and 30 sec) subs of the trapezium. I had stacked these separately. I just did a simple brightening stretch focusing only on the trapezium area. When it reached a brightness that was comparable to the central area of the longer exposure image I put it as a layer on top of this "main" image. A hide all mask was used and I carefully painted the mask to allow just the trapezium area to show through. The mask was feathered heavily if I remember correctly.
To specifically answer ok wx guy 24's dust question: I used the screen mask invert technique (see Scott's tutorial) to pull the faint dust out. It is a very useful tool. The use of masking is essential when using this technique.
Thanks again for all your comments.