"The event handler 'OnClick' is not allowed in this page."

upon further investigation, this occurs when a custom "system master page" for the site is used. I've used this for all other sites with no problem.

When using a custom master page you may need a page parser path to allow this type of behavior.

http://msdn.microsoft.com/en-us/library/ms562040.aspx

You will probably need to add your OnClick event handler as a delegate to the Click event on the link in your codebehind and remove the OnClick attribute in your .aspx page. For example, see below where the event handler is wired up to the event within the OnInit event handler.

Client Code (.aspx)
----------------------
<asp:LinkButton ID="authorize"  runat="server" >Authorize</asp:LinkButton>

Codebehind (.cs)
---------------------
namespace SPLibrary
{
    public class Default : Microsoft.SharePoint.WebPartPages.WebPartPage
    {
        protected LinkButton authorize;

        protected override void OnInit(EventArgs e)
        {
            authorize.Click += new EventHandler(authorize_OnClick);
        }

        protected void authorize_OnClick(object sender, EventArgs e)
        {
            LinkButton authorize = FindControl("authorize") as LinkButton;
            authorize.Text = "Label Changed";
            // required business logic can be added.
        }
    }

If you need to use client side script, remove the runat="server" attribute and you may need to change OnClick to OnClientClick.

Strange but the built in master pages from MOSS are okay, example blueband.master etc. There seems to be an error with mine.

Since I'm not a developer I can't try your suggestions. Because I will be modifying your AddProvProfWiz*.aspx page which has the Onclick event. This file is on several WFE machines

As I mentioned above a PageParserPath entry in your web.config file will correct this issue. Did you try this option?

<PageParserPaths>
 <PageParserPath VirtualPath="/*" CompilationMode="Always" AllowServerSideScript="true" IncludeSubFolders="true" /> 
</PageParserPaths>

After adding the above config to my web.config I was able to use my custom masterpage..