Thanks for your reply, I am afraid we only remember the dates of the trip, we have no other information on the visa as it was long time ago and we do not have the involved passport anymore; is that an issue? I am not too sure exactly what kind of informations they require about this visa (for example, we do not remember the date of expiration of the actual visa).
EDIT: I just found online that they seem to required the dates, visa classification and visa number. We have only the dates so I do not know what to put in if we unlock. The visa classification does not exist anymore as now ESTA is the new regulation and we do not have the visa number anymore :/
Should we unlock it but again we are afraid of some delay in processing (though we have a bit of time as case EU292XX)? Do you think that question is delaying the process especially as it is concerning my partner, the main applicant?
And yes haha sorry 'partner' is not the accurate term, we are indeed married under the UK law (our country of residence), though not in France (our country of citizenship). Again thanks a lot for your help and advice, much appreciated!