Right, ok, so one of your ex husband's creditors has registered a charging order. A charging order is essentially a second mortgage on the property.
This means that they can apply to court for an order for sale in order to recover the debt, unfortunately.
I suggest that you contact the creditor to find out how much is owing, and of course your ex to see if there is any chance of him clearing the debt.
A lot of the time, the creditor doesn't apply for an order for sale straight away, they should try to negotiate payments with your ex, and the charging order is just there as a belt and braces.
The way you describe your holding of the property, you are joint tenants. You should probably sever the joint tenancy now, as apart from anything else, if you die, your share will pass to your ex husband which obviously you wouldn't want.
You can sever the joint tenancy very easily; either you can both sign a form SEV and send it to the Land Registry, or if your ex won't agree, you can serve a Notice of Severance on him and then notify the LR. It's slightly more complicated to do it the latter way, but not hugely so,