I'm fairly certain it had been absoutely fine since it was fitted - the order of the kit going in went
RCD510DAB > Fiscon > MDI > RNS510DAB (replaced RCD) and I wonder if something had gone on the fritz at that stage.
I noticed that when I reconnected it all, I'd lost Fiscon and Nav from my Maxidot menu. Rebooting the Columbus brought back the nav and then the next time I started the car, the Fiscon menu came back, so happy for now.