Just a quick update. Got a reply from CAA FCL who are happy to process my verification - SRG 2110 - £44 fee - bring ID etc but cautioned that I best contact FAA Oklahoma for their input.
Seems like it should be no problem - they will action the EASA licence number and address update when received and automatically issue a 61.75 to replace my old certificate as a matter of course.
Now for a trip to Gatwick.....