Updating a customer through web service, not setting currency code

I am attempting to create/update a customer in GP2010. I am using Progress Openedge 10.2B to create the XML file which is then passed through to the UpdateCustomer routine in GP. The following is set in the xml to update the currency code on the customer/debtor.

<s0:CurrencyKey>

 <s0:ISOCode>GBP</s0:ISOCode>

</s0:CurrencyKey>

where the s0 namespace is set to point to "schemas.microsoft.com/.../01" using the legacy endpoint.

I have tried using the CompanyKey/ID tags within CurrencyKey to specify the company to validate the CurrencyKey against but nothing try will update the customers currency code.

Additionally, if I try to send an ISOCode that does not exist in GP no error is returned to say that is does not exist.

Any ideas what I need to send to update the currency code please?