Thanks Paul,
I wasn't using a sunset version, but anyway I upgraded to 4.2.2 and changed to v201302. After going around the garden and changing a few things, I ended up with exactly the same error. Its quite a long script that has worked absolutley fine in the past and just basically runs through a series of calls to create a campaign, adgroups then ads, then keywords.
I am working through the ad creating to see why it falls over halfway though.
btw, how can I get a heads up when libraries change etc? I only ever find out when I get a problem.
Thanks