hello Toby,
I was working on a large size project so wont be able to port whole project here but following is an example.. please check if it helps. Key is give ValidationGroup same to the validation controls and button.
Example of Control... check the ValidationGroup parameter
<telerik:RadDatePicker ID="PaymentReqDate" runat="server">
<Calendar
ID="Calendar1"
runat="server"
UseRowHeadersAsSelectors="False"
UseColumnHeadersAsSelectors="False" ViewSelectorText="x">
</Calendar>
<DatePopupButton
ImageUrl=""
HoverImageUrl=""></DatePopupButton>
<DateInput
ID="DateInput1"
runat="server"
DisplayDateFormat="M/d/yyyy"
DateFormat="M/d/yyyy"></DateInput>
</telerik:RadDatePicker>
<asp:RequiredFieldValidator
ID="RequiredFieldValidator3"
runat="server"
ControlToValidate="PaymentReqDate" ErrorMessage="Please
Enter The Payment Required Date" ValidationGroup="abc">*</asp:RequiredFieldValidator>
//Example of Summary...
<asp:ValidationSummary ID="ValidationSummary1" runat="server" HeaderText="Rectify
These Errors" ValidationGroup="abc" />
//Example of Button
<asp:Button ID="btnSubmit" runat="server" Text="Submit" ForeColor="#4c79a6"
ValidationGroup="abc" onclick="btnSubmit_Click" />