Got it working, heres the code if anyone has a similar problem

<%@Import Namespace="System.Web.Security" %>

<form runat="server">
        <asp:TextBox id="instId" style="DISPLAY: none"
runat="server">53352</asp:TextBox>
        <asp:TextBox id="cartId" style="DISPLAY: none"
runat="server">36901</asp:TextBox>
        <asp:TextBox id="currency" style="DISPLAY: none"
runat="server">GBP</asp:TextBox>
        <asp:TextBox id="amount" style="DISPLAY: none"
runat="server">30.00</asp:TextBox>
        <asp:TextBox id="testMode" style="DISPLAY: none"
runat="server">100</asp:TextBox>
        <asp:Button id="btnSubmit" onclick="btnSubmit_Click" runat="server"
Text="Buy This"></asp:Button>

    </form>

Sub PostData()

dim page, wpSignature, wpSignatureFields as String

        wpSignatureFields = "instId:cartId:currency:amount:testMode:Addres s1"
        wpSignature = FormsAuthentication.HashPasswordForStoringInConfig File("jamal" & ":" & instId.Text & ":" & cartId.Text & ":" & currency.Text & ":" & txtTotal.Text & ":" & testMode.Text & ":" & PostCode.Text, "MD5")
        wpSignature = wpSignature.tolower()
        page = "https://select.worldpay.com/wcc/transaction?"
        page = page & "instId=" & instId.Text
        page = page & "&cartId=" & cartId.Text
        page = page & "&currency=" & currency.Text
        page = page & "&amount=" & txtTotal.Text
        page = page & "&testMode=" & testMode.Text
        page = page & "&PostCode=" & PostCode.Text
        page = page & "&signatureFields=" & wpSignatureFields & "&signature=" & wpSignature

        Response.Redirect(page)

End Sub