Thanks Michel, that’s much appreciated! I didn’t know about that page for DUNS numbers. In fact I did have to deal with it back when I registered for the App Store. I will also make sure our phone number is still publicly accessible.
I found this page on the MS site that lists some certificate authorities if it can be of help to others.
Not sure what Thwate are like, but I think they are a bit cheaper than DigiCert but have no experience of their vetting process.
Thawte redirects to DigiCert. So that’s not going to be cheaper. 
Ah, I was looking at a reseller who are offering Thawte certificates at half the cost of their DigiCert pricing.
Update: Sectigo finally completed the verification, issued the certificate and apologised for the inconvenience. More grey hair, but at least I can ship the product now.