Gary Delafield

Dear HMRC I currently have a property on the market where my buyer has pulled out. If I proceed and buy my next house I end up with 2 properties in the short term and an overall chunky mortgage. My understanding is if my current property sells within 9 months there is no capital gains to pay (money would be used to reduce mortgage on the second property - which would then be my only residence) What happens if my original house as an example took 15 months to sell - do I have to pay capital gains? Thanks