What's going on here?

[FairfaxAikman]

It doesn't match the description of sleep paralysis though it does happen when I'm dozing in bed in the morning.

It also happens occasionally.

Basically I'll be lying dozing (having previously woken fully) and breathing through my nose.

Suddenly breathing will become really restricted, like my nose is blocked, but I'm unable to open my mouth to breath through it, not can I move my arms to open my mouth with my hands.

After a few seconds it "releases" and I can open my mouth to take a much needed deep breath.

While I'm paralysed it's difficult to keep from panicking, which makes the breathing difficulty worse. I'm worried that one day it will go on longer and longer.

is it some form of panic?

remember to breath, 11 in, 7 out

No. I only panic because I can't breathe, rather than can't breathe because I'm panicking.

It's like I'm being suffocated

but you are concentrating on it.
try to control your breathing

I do. I try to focus on opening my mouth so I can breathe. As soon as I can get my mouth open I'm fine.

You should see your GP who will refer you to an ENT specialist. Like you say, so far it's only lasted a few seconds but what if lasts longer at some point? You can't take that risk. Go and get checked out. Your anxiety will get worse the longer it goes on too and could lead to you being too scared to doze and cause sleep issues as well