OK, seem to be getting somewhere now 
As stated in the OP I work in 64 bit Win 7. I have a win 7 32 bitpartition on the same machine I use for various things and also for testing Lightroom updates without upsetting my main LR work partition.
Earlier tonight I isntalled 4.1RC2 on my win 7 32 bit partition and ahve been working on a shoot. Interestingly the RAN usage seems much better managed. Instead of progressviely creeping up to a point of ARRGHHH!!!, with RC2 memory is released when switching images. Great news!! Now bearing mind I still need to stest this in my 64 bit martition and I will isntall RC2 once confident it wont' fall right over, but positive os far.
ALSO, which I've just started testing is the negative cache disablement as described by Adfoibe Employee Julie Kmoch here
http://feedback.photoshop.com/photoshop_family/topics/lr4_0_reacts_extremely_slow
who said:
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Here's an update on what changed in RC2 relative to Develop performance.
For starters, in RC1 we experimented with turning off sharpening while sliders were moving. We got a fair amount of negative feedback on that, and have reverted that behavior. Instead, we moved more of the rendering to a background thread, which keeps the sliders moving smoothly. One caveat with the behavior in RC2 is that those background renders can pile up if you're moving fast. The final 4.1 release will do a better job of trimming these when possible.
I worked with a number of you who volunteered to try an experiment a couple weeks ago that turned off something we call negative caching. The results were mixed; a couple people said it was a clear improvement, others didn't see a benefit. But I'll offer it here in case it helps others who find performance starts out reasonably then suddenly, consistently goes south until a restart.
What the following will do is turn off a cache which saves some of your most recent work done in Develop such that if you revisit a recently touched image, it loads faster. However, if our calculations are off, this cache can sometimes get too big and cause ACR to use virtual memory instead of RAM.
To try it:
1. Create a text file called “config.lua” and put the following line in it: AgNegativeCache.enabled = false
2. Launch Lightroom, go to the Preferences dialog, Presets tab, and hit “Show Lightroom Presets Folder”.
3. Close the Preferences dialog, quit Lightroom.
4. Drop the attached config.lua file into the Lightroom folder that was opened in step 2 (do not put it in one of the preset subfolders). So the path to the file will look like this:
Mac: /Users/[yourname]/Library/Application Support/Adobe/Lightroom/config.lua
Win: C:\Users\[yourname]\AppData\Roaming\Adobe\Lightroom\config.lua
You’ll know this switch to turn the cache off is working if you see the “Loading...” symbol even when you revisit the previously edited image in Develop. (When caching is on, you can often revisit a recently edited photo without the Loading warning showing.)
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I'll report back laters
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