OK I managed to get this working by adding additional membership and profile providers to my web.config, including an additional web service property with additional provider attribute :
<membership>
<providers>
<!--(website provider here)-->
<!--additional web service provider-->
<add name="WebServiceMembershipProvider"
type="System.Web.Security.SqlMembershipProvider"
connectionStringName="ApplicationServices"
enablePasswordRetrieval="false"
enablePasswordReset="true"
requiresQuestionAndAnswer="false"
requiresUniqueEmail="true"
maxInvalidPasswordAttempts="5"
minRequiredPasswordLength="6"
minRequiredNonalphanumericCharacters="0"
passwordAttemptWindow="10"
applicationName="MyWebService" />
</providers>
</membership>
<profile>
<providers>
<add name="WebServiceMembershipProvider"
type="System.Web.Profile.SqlProfileProvider"
connectionStringName="ApplicationServices"
applicationName="MyWebService"/>
</providers>
<properties>
<!-- example website property -->
<add name="website_property1"
type="string"/>
<!-- example web service property with additional provider attribute-->
<add name="webservice_property1"
type="string"
provider ="WebServiceMembershipProvider"/>
</properties>
</profile>
<roleManager enabled="true">
<providers>
<!-- (website provider here) -->
<!-- web service provider-->
<add connectionStringName="ApplicationServices"
applicationName="MyWebService"
name="WSAspNetSqlRoleProvider"
type="System.Web.Security.SqlRoleProvider" />
</providers>
</roleManager>
Then in codebehind :
Membership.ApplicationName = "MyWebService";
MembershipUser user = Membership.Providers["WebServiceMembershipProvider"].GetUser(serviceUsername, false);
profile = ProfileBase.Create(serviceUsername);
profile.Initialize(user.UserName, true);
var myProperty = profile.GetPropertyValue("webservice_property1");
hope this helps someone else!