Thanks for all the replies, everyone. I checked out the Tom's Hardware and Overclockers sites - very handy. I've just had the case off the machine again and re-routed a couple of ribbon cables to help with airflow etc.
After powering up, I noticed that the PSU fan was blowing and not sucking! Which is always bad, as we know - possible damage to hard drives etc, ooo-eerrr matron, etc.
So anyway, I flipped the fan over and have now got some actual airflow through the case, and CPU temp down to 61C idle / 66C loaded. I've also fitted, but not connected, an intake fan to the front bottom (ooo-err etc, see above), so I'm now going to plug that in and see what happens. I'll let you know...
It turns out to be only a 250W PSU, so I think I'll upgrade that, in the next couple of months or so.
Cheers,
cbl.