If you live close to Northampton, you might like to either visit the main library or the local records office.
At both you will be able to check the parish records for Irthingborough. You should then get Joyce's parents - if, of course, you find her.
You could even find her marriage in Wellingborough - you will need to check which parishes that encompasses,. That will save you buying the certificate.
Also, it may be worth posting a query on the Northamptonshire board for a look up, as I believe some members have access to the Northants parish records.
