Ok… So… Here’s the answer…
After digging even deeper and quite literally pulling some of my contacts out of bed (thanks, much appreciated!) I’ve figured it out.
You need to visit the Post Office because this year, these Vouchers are being issued as Post Office Energy Vouchers rather than using the Paypoint system of previous years. OVO isn’t the only supplier doing it this way. Going to Paypoint will NOT work this time around.
You need to do what the letter instructs you to do - if the postmaster messes up, you need to take it up with their manager rather than OVO as they’ve all been told to process these.
Any Post Office staff who either reject the voucher by redirecting you to Paypoint OR just giving you the cash aren’t following the proper process for Post Office Payout - they risk not getting reimbursed for it as well which could unbalance their books.