There must have been a County Court Judgement against you before they got the charging order. And the terms of that judgement must have been breached. It may be that it was a "forthwith" judgement - i.e. pay the whole debt immediately - or an instalment order you could not afford.
Whichever it was, you could have gone back to Court for a redetermination but it is now too late to do that. However, you might still be able to apply to Court for a Variation Order to try to get an instalment payment order that you can afford. You need to take advice on whether this is possible at this stage - as Ethel says, see a solicitor or CAB. If you get such an order, then provided you comply fully with it (do not miss a payment by so much as one day) they cannot go for an order for sale.
All the above assumes the debt is solely yours. But you say "we bought a car". If the HP was in joint names then they should be chasing your ex as well as you.