I think I know the answer but just to confirm
If I make a virtual card and use it for a free trial and auto delete the card, that would stop any annoying auto renew charges working at all right? It won’t somehow pull money from another pot/main account
Correct. They have no way of knowing your other card details.
Perfect, many thanks 
It would still be worthwhile setting a reminder to cancel your trial near the end.
As above stated it’s still important to cancel it yourself.
Some company’s actually have T&C that makes it where the payment fails you still owe them the money as the only way to cancel it is by actually cancelling it.
But regarding your actual question, yes. Your details are kept fully protected - I use my virtual card for most of my online transactions for this very reason at keeping my details hidden.
I’ll definitely still make sure to cancel in time :). Just more of a fail safe
As @Ordog says - cancel your trial with the provider before the trial expires too.
Consider that you don’t delete a VC - it just gets archived. There is an audit trail of you owning said card and any transactions made using it. So if some agreement is broken, there is a trail of financial transactions available in any official disputes lodged.
It’s definitely worth doing to be safe. You probably won’t even need to delete the card especially if it’s linked to a pot as if theirs nothing in the pot then no payment can be made.
I’d speak to the provider first to make sure they confirm it’s cancelled, then you’ve a better chance getting your money back.
I’d personally never sign up for something I didn’t trust 
Never used a virtual I don’t think.
Even with things I trust I still use virtual when shopping online, mostly as my paranoia thinks I’m safer in the event of data breaches or in the event someone gets access to the account where the card is stored such as Amazon. Then they can only use what’s in the pot.
I tried something similar last month. What actually happened was that the company kept retrying the payment every two days triggering a notification to my phone and a feed item in Monzo every time it failed.
I ended up logging in and cancelling myself.
To clarify, my objective is a fail safe if, as has happened on several occasions, I set a trial to non auto renew and they do it anyway.
ExpressVPN did it two months in a row in fact. No longer needed it so turned off auto renew. Got charged. Spoke to support to get a refund etc. month later, they did it again.
Back in the day for me it was creditexpert… they repeatedly sent me letters and emails confirming they had cancelled the account, and repeatedly took the money anyway. And because it was a continuous authority I was hosed… Halifax wouldn’t stop it - not their problem apparently - and if I changed my card number they’d tell creditexpert the new one!
I don’t expect those kinds of problems from Monzo though, they seem somewhat sane.
Did you not chase to get the money back, if they sent you letters saying they’ve cancelled after you requested they shouldn’t be taking payment and I’ve never heard of a bank giving card details to a third party just because you “owe” them.
That seems like a big regulatory risk in my opinion
I think handing updated details to traders with a continuous authority is still a thing, although IMO it shouldn’t be without explicit permission. Not being able to cancel was such an issue the FCA changed the rules in 2013 giving customers the right to cancel them through the bank (and even issue refunds for the unauthorised payments).