Buying road tax question

[Kiltie]

I have to get a tax disc for my elderly mother's car (she's long since stopped driving).

AsMID indicates that the car is uninsured (not possible to just ask her if the car is insured - things a bit difficult just now).

I had it MOTd (yesterday).

If I go to the post office with my own insurance certificate for my own car (which says I'm insured 3rd party for any car not owned by me etc.) - will they be ok with that and be able to tax the car for me?

Thanks.

[Kiltie]

Thanks folks.