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Many thanks
For anyone interested this was the relevant text from Yahoo support....
"In relation with the problems with the URL when you click on the ad, we would like to inform you that is due to the Tracking URL tool. Tracking URLs may not be appropriate for all advertisers. While Tracking URLs can work with a broad set of technologies and platforms, we recommend that you test your links and closely monitor your traffic after turning on Tracking URLs.
Here are some reasons why Tracking URLs may not work:
The destination URL includes a parameter with the same name as a Yahoo! tracking parameter (such as OVKEY, OVRAW, or OVMATC).
The resulting URL may be too long. Adding this extra information may exceed the URL length that your system can handle. (The URL limit for ads served by Yahoo! is 1024 characters.)
Your system may only function with a single source variable.
Your URL may require "non-standard" formatting to work. For example, your system uses a "#" symbol instead of a "?" to indicate the start of the query string. Check with your ISP or webmaster if you are unsure of your formatting requirements.
Therefore we will advise you to turn off the Tracking URLs tool in your campaign. In order to dothat please follow these steps:
1.Click the Administration tab.
2.Click the Tracking URLs subtab.
3.Click the Tracking URLs off button.
4.Click Save Changes."