If your payments are all UTD then no chance
but on the other hand if you are on their 'special treatment' list then pretty good - especially near year end when they're all trying to balance the books!
Either way you can only ask but make sure you word your offer correctly - once they get the scent of moolah it'll be yoicks and tally ho!
Do you have assets Ie. own your own property? If not you could afford to really put the squeeze on them for a year or so and they will soon come round and accept. If you do however it is a different matter and you have to be aware of a posible Charging Order on your property.