>Thanks Viv, this isn't even a Web app it's a WCF service which I use to get data from a database and my client is a Winform app - I suppose the required assemblies must be in the GAC on my dev laptop - thanks again

If you are forced to use that assembly, I would not write over the GAC but simply drop it in the Bin directory of your application. By that, you control all exception libraries that are needed for your application. Later on, if you find a way to avoid it, you will simply be able to remove it from there.