Frederick's birth certificate would give you his exact date of birth, which might help in finding him on the 1939 Register and perhaps in finding his death.

Thanks kiterunner. Any ideas why they don't post the certificates within a few days (Covid-19 aside). I am sure I read they're dispatched 15 days after order placed?

Well, you should be able to order a PDF of Frederick's birth cert, so they won't need to post you an actual certificate:

https://www.gro.gov.uk/gro/content/c...ates/login.asp

but they aren't doing the same for marriage certs yet.

The information for ordering a marriage cert says "Standard - £11.00, despatched on the 4th working day from receipt of order." or "Priority - £35.00, despatched next working day from receipt of order." (Though obviously there are serious delays at the moment, and the site says only to order if it is urgent.)

aha - ok thanks