Well, I flashed and re-flashed all three available versions of the available firmware (which is difficult as the device won't flash while the problem is in effect) and still experienced the problem.

After getting mixed signals from Nokia 'care', I contacted Misco, where I got it from, yesterday and they sent someone to pick it up today. They also sent someone with a replacement one ... which arrived today. The extra bit they charged more than Play was well worth a) them having it in stock when I ordered and b) this level of swapout policy

I've been playing with my new unit (which says 2011 on the PCB) and not had a problem yet! My case definately looks like a hw related problem as I could reliably get the old one to fail to boot within half an hour of trying.

Let's hope that this isn't my second 'spoke too soon' moment in this thread.