Sounds like you got hacked. Work your way through these resources and follow all instructions to completely clean your site or you may be hacked again. See FAQ: My site was hacked « WordPress Codex and How to completely clean your hacked wordpress installation and How to find a backdoor in a hacked WordPress and Hardening WordPress « WordPress Codex.
Change all passwords. Scan your own PC. Use http://sitecheck.sucuri.net/ before and after.
Some plugins might be useful: Search for “security” « WordPress Plugins
Tell your web host you got hacked; and consider changing to a more secure host: Recommended WordPress Web Hosting
If you can’t do the work yourself, consider looking for a reputable person to fix it correctly on freelancing sites such as Elance. (It’s not a good idea to respond to unsolicited emails from forum users offering to work for you.)
Songdogtech
Thanks for your advice. I will endeavour to follow the instructions to the letter as soonas time allows.
Out of interest, after I posted this plea my site became logged out and all the offending adverts had gone.
In your experience does this usually require further attention? My point being is it highly likely that the offending malware is still active in my wordpress account?
I would suggest that you still proceed as if the site had been hacked. It takes a lot of work to ensure that all of the hacker’s scripts & back doors have been removed. If you fail to do that work, the hacker will just walk straight back in again.
