Understood and thanks for the feedback. I can see that might be the issue to avoid duplicate identities etc.

I'm only guessing at this point, but there is an option in the menu at the bottom of the app, in "gestiones" to "revocar el certificado". I honestly don't know, but might be an option worth looking at. It all depends on your level of expertise.

There is an option is to register via a video call, but I'm not sure if already having a digital certificate will make that difficult too.

If not, sounds like you might not have any choice but to make that trip.

Whereas if we could buy a card reader and use TIE.....

Round here ajuntaments only offer idCATs which is the same thing but only works Catalan websites which is why I needed to book a time at Social Security. I always thought you needed something more than the reference number to able to make changes to the declaration though, but I just checked the help and it says it can be done, so that's something I've got wrong for years.

To revoke it you need the digital certificate in the first place, or make a trip, or ring them up. I guess I could ring them up to revoke it then try getting a Clave PIN but as coverage at home is practically non-existent I thought I might as well get a certificate.

The reference number option is perfect if you just want to complete your tax return. All you need is the figure from the previous years tax return in box 505. (I'm not sure if it is important, but I always use the full amount and separate the euros and cents with a comma) Very straightforward and gets you into the draft return straight away.

Couple of tips that might help anyone get their tax bill down, check out your local allowances on house repairs and medical expenses:

We had a new roof put on the house, and in Cantabria we were allowed to offset that in tax over 3 years.
This year, my partner had teeth implants done, that is tax deductable for 10% (up to 500 euros max)
Medical bills can be offset as well.