>>> I have been told they can't go bankrupt because it is below £10,000
Wrong. Another person can't make someone bankrupt if the amount owed is less than £5000. There's no minimum for someone to declare themselves bankrupt. (However it costs £680 to do so, so people aren't going to do it for a few pounds).
>>> If I knew who their new landlord was I would definitely contact them
If you can find out their new address you can then find out who owns that property by paying £3 for a copy of the title register here:
https://eservices.landregistry.gov.uk/wps/portal/Property_Search