Hi @Ross (EdiKUG) - really sorry I only just saw this. I think one major factor is that a rather large organisation (SkyBet and Pokerstars) in Leeds uses Kotlin, and there's quite a lot of Kotlin devs for that reason. Most things that were Java are now Kotlin there (I also work there). I think maybe 50% of the devs were SkyBet or Pokerstars employees.
Other than that - I have also worked with a local recruitment company and helped them with an event called The JVM Thing, which we set up because leeds didnt have a Scala/Java/Kotlin user group, but we thought being polyglot was better... anyway - they have a lot of linkedin and twitter followers, and they were happy to sponsor and promote the event (although we dont share data with them)