Hi Zaph,
I don't have any experience with that radio system in particular but generally you can't mix and match receivers and handsets. Only certain pairings work. You also cannot change band width as this will be set when you turn on the handset. Interference used to be more of an issue on older radios, and moving the aerial away from the motor should help. Is it sticking up?
Maybe something in the hall itself is causing issue. I would expect high end radios to suffer less, but I myself use fairly cheap ones and rarely have issue. Recently I attended a meet in a hall and believe it or not I did have interference (only slight) when driving past a particular chair.
If you have another handset and receiver combo, it might be worth a try, but it could be the same. Maybe post a photo of your current receiver setup?