Although the rent you pay will be based on the market rent of the home, it will be discounted by 10% from the market - and will be further reduced to reflect your investment in the house. For example, if the market rent on the property is set at £1500 per month, the base rent for the home is set at £1350 per month. If you invest 10% in the property, you receive a further 10% discount on it - and the net amount will be £1215 per month.
If you staircase up by purchasing loan parts on the OnStep secondary market, you will receive the rent back for the loans you hold. This will in effect reduce the monthly rent you pay. This will happen from the day you staircase up.
The rent will be fixed for the first two years and then adjusted at the rate of inflation annually thereafter. For example if your starting rent is £1215 per month, inflation rate is 2% and you do not staircase up; your rent for year 3 of your residence will be £1239 and your rent for year 4 will be £1264.
The annual increase in rent will be in line with the annual change in an inflation index, known as CPIH. CPIH is the consumer prices index (with housing) maintained for the government by the Office for National statistics. You can find out more about it here (https://www.ons.gov.uk/datasets/cpih01/editions/time-series/versions/4).