If you are serving a prison sentence, you can apply for any of your fines, enforcement warrants or court fines to be converted to run concurrently with your existing sentence (see “Other factors that can affect a sentence” in Imprisonment and prisoner rights).
You can convert unpaid fines, court fines and warrants from the time you entered prison (i.e. including time on remand) to run concurrently with your time in custody (as long as your sentence is for an offence that is non-fine related). You can apply retroactively at any point while you are in custody.
Further, if the period of imprisonment under the unpaid fines exceeds your sentence, then the court has additional powers to deal with the remainder of your unpaid fines, such as discharging them in full or in part, imposing a fine default unpaid community work order, a time to pay order, an instalment order or adjourning the matter. Additional prison time remains an option; however, a court must not make an imprisonment order if it is satisfied that you did not have the capacity to pay the registered fine or you had another reasonable excuse for the non-payment. Finally, the court must not order imprisonment unless satisfied that no other order is appropriate considering all the circumstances of the case. If additional prison time is ordered, this is to be served cumulatively to your existing sentence.
If you are in prison for a fine-related sentence, the court has additional powers to make orders to deal with your unpaid fines as noted above (e.g. discharging in full or in part, imposing a fine default unpaid community work order, a time to pay order, an instalment order or adjourning the matter).
Note that if the court orders a term of imprisonment, this is served cumulatively to your existing sentence.