Thanks guys

Yes there is still a little flicker, most noticeable in the clouds. Their seems to be a slight change in the structure of the cloud in the illuminated area at the top right of the frame in the latter part of the animation. The actual "tonality" of the clouds seems to be steady with the fill light setup, I'm no longer seeing the whole sky area changing tone on a frame by frame basis.
I'll try rendering with the Acceleration cache turned off. Is this done by setting the Acceleration cache to none? At the moment each frame is taking about 10 min to render, it'll be interesting to see how much extra is added to the render time.
(ok I'll see how long I can hold out, lol, its now rendering at 30 min a frame!)I was worried about the EXR rendering to start with, I had assumed that Photomatrix would be able to batch convert them and run its rather good "Tone mapping" tools on them in batch mode. Unfortunately Photomatrix wont load EXR files in Batch mode and I didn't fancy manually converting 300 files

Photoshop CS3 will open them but I couldn't figure out how to get it to batch convert with an action script (however I'm sure it can be done). Finally I just fired up Afterfx and to my surprise it loaded the EXR easily, although I couldn't find any "tone mapping" settings in AE.
Thanks again
Richard
ps: the render settings are Detail 0.8, AA 8, GI (0,0). Cloud: quality 0.9, samples 626. Atmosphere: samples, 128
pps:
The HQ Quicktime (193MB) can be downloaded at the bottom of the page