To test the fixed disk theory, I tried using a pendrive. This works sucessfully, which blows that theory out of the water.
I then got to thinking it must be because the ramdrive isn't active during bootup, so I switched the system off (cache working on pendrive at this time), removed the pendrive, re-started and then after boot and logon, reinserted the pendrive. The os will still write to the pendrive at this stage.
I'm kinda running out of options now, but the only thing left really is that the ramdrive is a virtual drive, everything else is hardware based.
Are there any settings/registry hacks/scripts etc to allow the cache to be written to a virtual disk. Of course, I may be wrong about this also, I really am groping in the dark, and would really appreciate some advice if anyone has some.