What would happen if?

You owe betfair 4 grand

No you would owe them 8k when they link the two accounts

it  plays tricks to your mind when that happens  win at 11.55 pm on sunday then lost the lot at 12.20 am on monday and a big hole on wed   aaaaaaaaggggggghhhhhhh

"You owe betfair 4 grand"

Really?  Betfair don't give credit.  It is a deposit account and you can only lose what you deposit.

Could you ever imagine the case going to court?  Well sir, you deposited £5k to bet with and placed two bets, On the first one you won £10k  and on the second you lost £15k, therefore you lost the £5k in the account.  The plaintiff is asking for another £4k on top!  Case dismissed.

I believe they remove it from your bank account.

I'm aware of people who have tried to remove all the money from their account so PC couldn't be charged only to have Betfair reverse (in full) the last withdrawl.

you didn't deposit 5k - you'd been here for years to reach 40%

Asparagus you haven't made a recent withdrawal for them to reverse, indeed you are a net loser in this scenario since premium charge was last paid so why would you?  You are talking about examples where people have took their money and run only to find Betfair take it back a couple of days later, not an example where you have subsequently lost the money that would have been used to pay it.  Maybe they would reverse the losing bet and pay premium charge out of your losses?

Still your account would be closed. Having won over £250k you'd want the option to use bf in the future, even if on losing run currently.

It's a charge for using the exchange set out in the T+C that you agreed to.  They ask you to pay the charge on Wednesday and if you don't pay it, decide whether to set the debt collectors on you.  Would love to see someone going to court over it though on the grounds that the charge was arbitrary and excessive.

In reality I imagine it's never happened, but if it did happen I imagine they wouldn't properly chase it as a debt.  The question is whether it's a legally enforceable debt in those circumstances?  Unlikely imo, because 1. It's a deposit account.  2. Bf could make steps to prevent it happening and 3. It wouldn't be deemed fair to charge someone extra on top of their losses.