I have elaborated. You need to use google a bit more and check out IP blacklists, port 25 testing etc. As I've said, you need to look at the logic flow here and work out which part of the chain is failing. You're a Zend Certified Engineer, you should know this!
Sendmail is an SMTP client. That means it will connect to a SMTP Server at the recipients domain and pass the email straight to it (in theory) OR it will be configured to use an SMTP relay. Now, it may be that your SMTP is not working because your ISP has blocked port 25 (maybe they've had a report of you spamming?). It maybe that your sendmail config has changed (eg, rolled back with a system restore) or maybe your IP is on a spammers IP blacklist. Again there is no definite answer I can give you because its a process of elimination.
You say you only need to send mail to yourself locally yet your log shows you're clearly trying to send to fletcher.co.uk which is not in your hosts file (so its out there on the web) so I'm not quite sure that I follow when you say you only need to send to yourself?
My helpful sig is on vacation trying to loose some weight. It got a bit fat and caused a few problems but it will be back at some point!