MOT exemption and road tax

[Lotobear]

I have a 1976 Alfa which has recently fallen into the tax and MOT exemption bracket. It was last MOT'd in 2017 and I had it on SORN until mid 2018

I taxed the car online during 2018 (nil cost) and took it off SORN. I'm sure I was prompted when doing that to declare the car eligible for no MOT which I did.

I've been driving it recently and when I checked yesterday online it came up as 'no MOT'. The Gov site seems to suggest that you need to print off form V112 and take it to a post office when you tax the vehicle (but why would I tax it a a PO when I can do it online?)

Can anyone clarify the process here in order to properly declare the car MOT exempt? Surely it's possible to do online... which I thought I already had?

Cheers