From OVO's website:

To pay your bill, you can also make a transfer to out bank. Our bank account number is 83683249 and sort code 20-00-00. 

Remember to include your OVO account number and postcode in the reference field, so we can link payment to your account.

Thank you for the update.  As I said I want to pay by CASH (the actual queens head paper stuff) not through my bank account (if I have one?).

I shall give it a go & see what fuss Barclays makes when I try this.  I am expecting  resistance.

You can pay cash over the counter at any bank into any bank account that you have the details for. I can't see why your bank would make a fuss.

@pen-nib You’d take your cash to your bank and fill in the payment slip as @M.isterW described. 

You obviously haven’t tried paying in cash to a bank account other than your own.  Give it a go as an exercise & you will see what I mean.  The banks band that practice a few years ago in case we are money laundering Oligarcks.  Everything is designed to make it as hard as possible to pay for anything using cash but I am resisting on moral & ethical human rights grounds.    

I’ve spoken to my bank today to check this over with them. For this purpose, I used a random bank branch of my bank that does not know me, because I’m too well known at my local branch and it would otherwise affect the answer. I didn’t give this particular branch my details.

They’ve told me that as long as you’re attempting to make a payment to a legitimate UK business bank account like the one OVO uses, they’ll happily accept it and process it for you at no cost and won’t get upset or fussy about it, even if it’s by paying in cash over the counter. The most likely time you’d trigger anti money laundering rules is if you’re trying to pay some random bank account that’s untrusted or is in another country. Even then, it’s risk based and not everything will set off the alarms.

For good measure, I also spoke to a random Barclays branch that doesn’t know me either (I don’t use Barclays at all anyway) and they told me the same thing as my bank did. You should be good as long as you’re basically not trying to throw tons of cash out of the UK all in one go.

Actually @pen-nib all our nearby banks have closed down anyway - you are right in that I haven’t tried it recently.