Just in case you haven't had this sorted already I thought I'd post my experience.
After unsuccessfully trying to get through to the tax office by phone for weeks, I emailed them giving my husband's and my PPS numbers and our date of marriage and said we wanted to be assessed jointly. Received an email a week later confirming that our status and tax credits have been adjusted - no mention of the marriage cert. Like you, we got married abroad and we haven't had the cert translated yet so I was delighted.
As an aside, there is no need to register foreign marriages here. The original with an authenticated translation will suffice.