To advise you properly, it would be helpful to see the payment agreement that you signed with the OR. The general rule is that OR shall check your circumstances, bank transactions, expenditures to create a right payment scheme for you. How rigorous are the checks depends on the behaviour of the person (the company director in your case), how severe the breach is (defrauding creditors, wrongful trading, non-compliance with court orders, etc). If you pay on time and OR has no issues with you, it is unlikely they will change their approach to the assessments.
Without seeing their calculations it is hard to comment why the total amount of debt you are paying out is bigger than the initial amount of debt. They may have added their fees on the top and the lender’s unpaid interests as well. You may always send them an email asking to explain to you why the calculation is that much. If you have financial difficulties (lost your job, for example) please tell them immediately so that the payments may be reduced.
Do not forget to declare the following expenditure: rent or mortgage payments, food, heating and lighting, TV licence, broadband and telephone service, household insurance, car tax and insurance, if OR has allowed you to keep your car, membership of the AA, RAC or similar, if you're allowed to keep your car, professional membership fees that are part of your job and not paid by your employer, prescriptions, dental treatment and opticians' fees, payments under a maintenance order or child support agency assessment, a reasonable monthly cost of a mobile phone, dry cleaning; plus reasonable amounts of spending on: clothing, holidays, hairdressers, extra-curricular activities for your children, after-school clubs, pets.
A payment scheme usually lasts for 3 years only. There must be serious reasons to extend it for longer.
Please let me know if I have answered your query and if you have further questions.