Hi Niel, thanks for posting.
The ?úx amount per month that has been agreed should really be the amount that is left over once your living costs are met - meaning you shouldn't have any spare money to save up anyway. Although you will be paying out a lot less than before, I suspect that you may have been using credit to top up your income previously, which is what commonly happens if someone has unaffordable debts. See how you get on, but you might find that once you can no longer "rob peter to pay paul" your budget may be a bit tighter than you think.
Regarding your overtime - this is something that would have to be agreed with your trustee. If you are regularly earning overtime then your trustee may require you to increase your monthly payments - though they would probably only ask for a portion of the extra money, otherwise there would be no incentive for you to put in the extra shifts in the first place!