Hi AlastairE, that you for great posts here!
Yes, 1.7 Mhz transducer disc doesn not work. I ordered one from China and replaced it today in my Dimplex with blue tank. It gives just 1/2 inch water hill with no mist at all.
Could you, please, advise how to adapt electronics from 2,2 to 1.7 Mhz...