Thanks for quick reply and direction, I've used your sample page to try to locate the error and this has highlighted that this is not an SSL problem as your page error trap works, sorry about that! The problem lies with:
<customErrors mode="On" redirectMode="ResponseRedirect" defaultRedirect="~/Account/Exception.aspx">
if I use:
<customErrors mode="Off" redirectMode="ResponseRedirect" defaultRedirect="~/Account/Exception.aspx">
the problem goes away - obviously.
I now realise I need to treat this HttpHandler error diferently to normal site errors as the upload page is getting confused as it is both trying to redirect to a specific upload error page from the uploader client code whilst also trying to redirect to
the standard error page as invokded from the server side error trapping, which also explains why the page was not redirecting anywhere at all!
If you have a pearl of wisdom regards this then I would appreciate that otherwise I'll plough on and see what I can come up with.
Thanks very much - it's a quaility component.