I think I may have answered my own question (and forgive me if my terminology is not right).
I'm using:

•     an old instance of Umbraco (4.11.3 but I'd imagine this would happen on any version).
•     WebForms (changed in the umbracoSettings.config)
•     a usercontrol written in VB

The steps to recreate my problem are as follows:

•     create a user control which includes a button, which when clicked, should update a label.
•     create a macro, which uses the usercontrol.
•     add the macro to a page in the Editor (i.e. to the maincontent field)
•     the template used by that page outputs two versions of the content (depending on the language selected)
•     this template has an umbraco:macro which includes a bit of razor logic
•       - if the language is English, output the English content
•       - if the language is Welsh, output the Welsh content

The problem was that the button didn't work if the maincontent field was output via the umbraco:macro (note that it was displayed).

It did work if the if I output the field containing the maincontent as an umbraco page field, or with .net logic to check the language.

It also worked if I put the macro directly into the template.

My question is why won't the button work in the umbraco:macro code which gets the maincontent field?

Is it something to do with postback, page events, and razor doing things differently?

The code is:

<%@ Master Language="C#" MasterPageFile="~/masterpages/MasterTemplate.master" AutoEventWireup="false" %>

<asp:Content ContentPlaceHolderID="ContentPlaceHolderMain" runat="server">

        <h2><umbraco:Macro runat="server" language="cshtml">@((Session["lang"].ToString() == "cy") ? @Model.pagetitle_cy : @Model.pagetitle)</umbraco:Macro></h2>

        <!-- This works -->
        <umbraco:Item field="maincontent" runat="server" />

        <!-- This works -->
        <% if (Session["lang"].ToString() == "cy") { %>
                <span>
                <p>Welsh</p>
                <umbraco:Item field="maincontent_cy" runat="server" />
                </span>
        <% } %>
        <% else { %>
                <span>
                <p>English</p>
                <umbraco:Item field="maincontent" runat="server" />
                </span>
        <% } %>

        <!-- This works -->
        <umbraco:Macro Alias="nptImport" runat="server"></umbraco:Macro>

        <!-- This doesn't work -->
        <umbraco:Macro runat="server" language="cshtml">
                @((Session["lang"].ToString() == "cy") ? @Model.maincontent_cy : @Model.maincontent)
        </umbraco:Macro>


</asp:Content>