OK, so I've spoken to my hosts and tried various things:
I did a clean installation of WP 2.0.2 (using Fantastico) in a sub domian and editing/publishing posts worked perfectly...until I imported my existing database, after which I got exactly the same error again. We tried repairing the database and still get the same problem.
My hosts suggest this means it is neither WordPress nor a hosting issue, but a database issue.
HOWEVER, when I remove my largeish list of pings (or just try with a couple of pings) the problem disapears. Which makes me think that what is happening is that WordPress is timing out whilst trying to ping my long list?
The only possibly illogical thing about that thought, is that with the clean fantastico install (before the databse was restored), WordPress would hapilly ping everything on the long list.
Searching the Fora, I did find this plug-in http://somethingunpredictable.com/wp-no-ping-wait/
Which seems to solve the problem (which would appear to support the idea that WP is timing out).
However, I'm not convinced that any pinging is actually happening with that plug in activated, as pings don't seem to show up on Technorati or elswehere, so I'm wondering if all it is actually doing with WP 2.0.2 is to switch off the pings, which is how it is solving the problem?
My hosting company have suggested batting it back to the WP fora, so does anyone have any comments/suggestions?
BTW it is not, as far as I can tell, a whitespace issue. I have doublechecked and reuploaded both files mentioned in the error message as well as the wp_config.php and it makes absolutely no difference whatsoever. I still get the same error messages.
It has been suggested the database may be corrupted in some way. If that's the case it means doing a clean install and manually copying everything across, which I'd rather avoid.
On the other hand, if the DB has been compromised, I don't want to use a quick fix like the plug in above or reducing my ping list, only to find somewhere down the line that I have to recreate the databse manually...by which time I may have added a couple of hundred more posts.