Typically Restons will go for a re-determination (appeal?) if the debtor has a house - they will request a forthwith order and then accept monthly payments with the proviso that they get a charging order.
However you already have an installment order and Restons it could be argued are abusing process as they don't like what the court has already done.
Not much you can do to stop them applying but defend the claim on the grounds that there is already a CCJ awarded. Also make sure that you carefully vet what they are claiming and dispute all their costs and charges if they are requesting them.
Restons have had more than their share of complaints for harrassment, they are also well known for going for charging orders and attachment of earnings and piling on costs. However they are only a DCA and they are governed by the law just like the rest of them.
If you are unemployed and have no assets they can place a legal charge on, you have no need to worry, just pay what the court agreed and tell Restons to get off your case with as few words as possible.
On the other hand it may seem that Restons think you are a long term investment for their greedy little paws. If you have a property with or without equity, it is another story. Restons will go for a charging order and once they get it will start charging interest so they will get their money plus interest when you sell the property. It should be against the law that an unsecured debt can be changed to a secured debt by a parasite who buys that debt, but at the moment it is legal and I doubt it will change.
As I wrote in a previous post, charging orders are being used much more by DCAs.
They are not just a DCA and do have a legal side but having said that can only act within the law.
You do not state whether you have a property - if you do as Duff says their intention is to apply for a Judgement Forthwith and then a Charging Order.(assuming you are unable to "cough up on the spot")
As you are paying back £20 odd on a debt of £3K I cannot see why they would be attempting to vary the order unless you have any assets.
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Considering, secured loans come with that warning about 'your property can be at risk' etc, why should any unsecured creditor use this method of heavy handedness.