So a friend gave me a voucher code today, I go to PS Store and it asks me to update it, which I did.
Then I try to redeem the code, it recognises it's a PS Plus subscription, then when I hit confirm it comes up with something like
"PS Store has encountered an error and has to close, try closing and re-opening the application. Use the PS button to exit the store"
Thankfully the code is still usable i.e. it doesn't say code has been redeemed when I try it again.
I've tried it on my pc, but it says this kind of code can't be redeemed on a pc.
So I've fixed the problem, without fixing the issue!
Successfully redeemed the code by going through account management -> Redeem codes, instead of going into playstation store.
No idea if other people will still have similar problems with PS Store or not.