Well I had to return my loco to the retailler this afternoon as it failed. Was getting movement and lights but no sound, but the roof of the loco was very warm. The loco had been running in for around 20 mins when the failure happened. No shorts, no stopping and no smells, just the sound went off.
The retailler checked the decoder on his decoder tester and it was fine so it appears the speaker had died.
The replacement sounds a lot better but the roof is still getting warm.