I honestly dont know how you dont live with overwhelming anxiety living with someone who is so reckless with money. I would be a wreck!
First of all, I hope he manages to send across the £2k you have requested - its important to at least get this back.
The remaining £3k, wait until the date he says he will have this back. If he doesnt have it back by then, you offer him a payment plan to pay back the money to you over the next year or so.
The thing is, he can either agree to send the £3k back/make repayments, or he may refuse.
Either way, if he refused to send or repay, I would be leaving him once I got my £2k back and cutting my losses, then working my ass off for my kids to gain back the £3k they lost.
If we agreed to a repayment plan, I would leave him once the payments were completed.
Obviously this man cant be trusted. He has broken your trust on what seems like countless occasions, on different matters.
You and the kids deserve better.