Using nodes:
DECLARE @xml xml =
'<Document xmlns="urn:iso:std:iso:20022:tech:xsd:pain.001.001.03">
<CstmrCdtTrfInitn>
<PmtInf>
<Dbtr>
<Nm>PAYER COMPANY</Nm>
</Dbtr>
<CdtTrfTxInf>
<Amt>
<InstdAmt Ccy="GBP">1.00</InstdAmt>
</Amt>
<CdtrAgt>
<FinInstnId>
<ClrSysMmbId>
<MmbId>123456</MmbId>
</ClrSysMmbId>
</FinInstnId>
</CdtrAgt>
<CdtrAcct>
<Id>
<Othr>
<Id>12429468</Id>
</Othr>
</Id>
</CdtrAcct>
<RmtInf>
<Ustrd>Billing</Ustrd>
</RmtInf>
</CdtTrfTxInf>
</PmtInf>
</CstmrCdtTrfInitn>
</Document>';
WITH XMLNAMESPACES('urn:iso:std:iso:20022:tech:xsd:pain.001.001.03' AS ns)
SELECT N.b.value('(../ns:Dbtr/ns:Nm)[1]', 'varchar(20)') AS Nm
,N.b.value('(./ns:Amt/ns:InstdAmt/@Ccy)[1]', 'char(3)') AS Ccy
,N.b.value('(./ns:Amt/ns:InstdAmt)[1]', 'money') AS InstdAmt
,N.b.value('(./ns:CdtrAgt/ns:FinInstnId/ns:ClrSysMmbId/ns:MmbId)[1]', 'int') AS MmbId
,N.b.value('(./ns:CdtrAcct/ns:Id/ns:Othr/ns:Id)[1]', 'int') AS Id
,N.b.value('(./ns:RmtInf/ns:Ustrd)[1]', 'varchar(20)') AS Ustrd
FROM @xml.nodes('/ns:Document/ns:CstmrCdtTrfInitn/ns:PmtInf/ns:CdtTrfTxInf') N(b);