Thanks for your enquiry.
Firstly, you only have a right to charge the interest rate as detailed in the Charge document. So, you will need to check the wording to see if default interest is mentioned and if so, what percentage rate.
Looking ahead, I would take action to repossess the property. You do not have to get permission from the main Mortgage lender that gave your Buyer his main mortgage before taking steps to repossess the property.
The first mortgage lender takes priority if the property is repossessed, but if there is plenty of equity, any money left over after the first Mortgage has been paid off will be used to pay you off.
Initially, I would suggest that you get a Solicitor to write a letter stating that unless the full outstanding balance is not paid within 14 days, then you will have no alternative but to issue repossession proceedings.
I hope this assists you and sets out the legal position.