I tried F6 originally (the latest BIOS) but went back to F5 to see if it made a difference, which unfortunately didn't help.
I've now moved the RAM from slot 1 to slot 4, so now have two 2GB sticks in slots 3 and 4. The BIOS and Win 7 now recognises 4GB total, but only as single channel obviously. Running a memory check now, but would seem that both sticks work OK, and just the slot they're installed in makes a difference. Kinda odd, and hoping it's not a sign of a faulty mobo (note: M.I.T reported that it could see the 2GB stick in slot 1, but it just wasn't enabled - no settings anywhere to change this either!)
If anyone else has any experience of this, would love to hear from you!
PK.