Following up on my posts about the problems I had playing DirectX games on the M1330 (Gaming Blues and More Gaming Blues) I finally caved in and actually called Dell last Wednesday rather than continue to wait for the Internet based ticket issuing system to actually work. Dell requested a little more diagnostic work ( facilitated by Mr Cronin, thanks Pat ) that involved swapping the hard drive out with that in a similarly configured M1330. That pretty much proved that the problem was with the hardware and Dell eventually arranged for a technician to come and replace the motherboard and graphics card. The delay was pretty much all my own doing in the end and to be fair to Dell once they accepted that the issue required a hardware swap they had a technician out to do the job within a day.
Dismantling one of these things completely is a reasonably complicated task - all in all it took the technician just over an hour to take everything out and put it all back together again, and he really did know what he was doing. Now that I've seen it I think I'd have about a 50:50 chance of repeating the exercise but I don't think I'll ever try.
Anyway the good news is that the swap has (so far) completely eliminated the problem so I can now happily waste hours playing TF2, Portal, HL2 and UT3.
The other good news is that the network adapter that I was disappointed to discover is only a 100Mbps part is a very easily replaceable (and upgradable ) part and the WLAN antenna cabling is kitted out to support 802.11n. So two of my (minor) beefs with the machine are fairly easily fixable should I choose to do so.
Edit: Just adding a link to my final update on this here.