Assertion SOAP Response Failed

I'm testing a web service deployed on a local application server. I've recently changed the message standard from SOAP 1.1 to SOAP 1.2. My test messages are still handled correctly by the Web Service
but when I try to use them in a test suite in soapUI 3.5 the SOAP Response assertion fails.

The new message envelope setting (Client name redacted in namespace.) is

<soapenv:Envelope xmlns:soapenv="http://www.w3.org/2003/05/soap-envelope" xmlns:v0="[redacted]">

and the response message starts with this:

<env:Envelope xmlns:env="http://www.w3.org/2003/05/soap-envelope">

soapUI fails the assertion with the following error
line -1: Element Envelope@http://www.w3.org/2003/05/soap-envelope is not a valid Envelope@http://schemas.xmlsoap.org/soap/envelope/ document or a valid substitution.

Any ideas?

Thanks.