Well – I installed the final release of SP1 last night with no problems so far.
I first had to uninstall the SP1 RC – and then I decided to run Windows Update – low and behold there were about 12 updates that I had been missing for the past two months.
After many reboots later installing the updates I fired off SP1 overnight (it takes a while). Woke up this morning with it all installed.
Run Windows Update again afterwards and I had one important update to install still from Oct last year ? Anyway all installed fine.