Hey again!
Thanks for all the help! My initial problem was slightly different than you interpreted. For some reason my code-window wasn't picking up the ajax tags in my subpages (even though all the configurations were right). I had to reset my toolbox and add the controls again, then it was picking up fine.
And the ScriptManager problem also took care of itself as soon as I did that. I now have an instance of the ScriptManager in the MasterPage and the Subpages pick this ScriptManager up (can't have two ScriptManager objects in one page, hence impossible to have a separate one in the Subpage).
The only problem this creates is that the AJAX tags (such as ) don't get automatically picked up in the Subpages, this means I have to handcode it out. It will work fine when the page loads because the ScriptManager is in the MasterPage. I now know the syntax for the AJAX tags, if you do not I suggest copying over the passages from the MasterPage, you will get the usual ASP.NET assistance in this class.
One of the obvious mistakes that I did was to think of the MasterPage as a separate page. I think it has been mentioned in this Forum time again: think of the MasterPage like a control. This really made a lot more sense to me.
Thank you again! I hope this solution can help other people facing similar issues.
Greetings, Peter.