(Several months later..)
Well, the good news. O2 has released a firmware patch which sorts out a lot of the issues I mentioned. After a hard-reset, and all of O2's own applications have installed, you're down to just 19 processes running (applause).
The problems I was experiencing with ActiveSync have finally been resolved by Microsoft. They've done an upgrade to ActiveSync v3.7.1, cleverly called it v3.7.1, as they don't reckon that much has changed, and now the Explore button works everytime.
The bad news...
After installing O2's firmware upgrade, autorun.exe doesn't EVER work !
The device doesn't try and kick off autorun.exe in EITHER the Storage Card or (built-in) "Storage" directories. Aaahhhh !
So the device is certainly a lot more reliable now than when I bought it, but O2 needs to put a bit more effort into their testing department...