I did a little investigation into Tuesday's early morning "attack" on my machine, and discovered that a total of 18 individual machines around the web were involved, including machines in Germany and here in the UK. I'm not too sure what to make of it, but I'd guess it was just script kiddies playing around.
I've come to that conclusion because even though port 2613 is a "well known port", it seems to me to be a very obscure port to try to gain access to. The port itself is something to do with the SMNTUBootstrap (what ever that is), but I can't find any information on it anywhere on the web.
Interestingly enough, port 2613 is mentioned in posts in the tw.bbs.comp.hacker newsgroup and in the alt.hackers.malicious newsgroup. I haven't had any repeat attacks since, so I guess Tuesday nights attack was just a one off - and my machine just happened to be the unlucky one in the firing line.