After thinking it was impossible to include .asp pages dynamically, I've managed to crack a way of doing it, which although technically not an 'include' method, uses the Server.Execute function to get round the problem as follows:
On the main page you wish to view, use the function Server.Execute "Folder/"&Variable&".asp" where you want it to appear - this will 'include' the relevant file.
On the page you are including, if you wish to execute asp within it, you can use the following method under some circumstances: Make sure that the variable is taken to the main page in a query string, ie the address off the main page will be in the format mainpage.asp?my_var=<%=my_var%>
On the included page, use QueryString = Request.ServerVariables("QUERY_STRING") This takes the variables from the url of the main page (eg my_var=<%=my_var%> in the above example). You can then chop this up so that the particular variable you want is taken for use in the include page (eg my_var).