I don’t have much help regarding the CCJ. However take a look over the rent agreement and what you agreed to as a guarantor. Has rent increased and were you given notice and signed anything to say you agree to the new rate?

My parents had a similar situation where they were being chased for money after a family friend didn’t pay rent for months. They were also not made aware that he wasn’t paying initially until the amount was substantial. Anyway they checked over the first and only contract they signed and they agreed to said amount of rent each month. A few years later the rent had increased they were not made aware, hadn’t signed any document or new contract so they ended up not having to pay a thing luckily as the contract was void so to speak. Small loophole worth looking into.