Thanks all for the suggestions, and the explanations. It's useful to know why removing the suppression components was suggested, but I think it's my last port of call. I'm ultimately looking to ensure this model is working correctly prior to selling it, and if I can't get it to work with DCC, then I will need the caps/inductors if it is to be any use as a DC only model.
I have the received the Zimo decoder that I ordered off the back of @Kaput's advice, and will try resetting the existing decoder, as well as digging out my Elite controller. I have some time today to try this. I'll let you know how I get on.
Cheers Craig