Hi MarkieMark,
If there is currently no arrears on your mortgage account, no other breach of your mortgage and you are paying your contractual monthly instalment, Start Mortgages have to honour your original contract with PTSB.
Should you default on the agreement then Start can take action, only legal action in your situation. I would suggest having your mortgage re-coded from BTL (buy to let) to PDH (private dwelling home). Typically you just need to provide proof you are living in the property. By law a receiver cannot be appointed to a PDH that you are residing in. Also having your mortgage coded as a PDH gives you the protections of CCMA should you have the unfortunate circumstances of going into arrears again. From the information provided no need to worry, just keep paying your mortgage.