Thanks Kurt.

I have about 1 inch of gravel , increased to 2 inches on the drier side, this is topped by mesh, then the soil.

It smelt just really sort of boggy! Sorry, I don't know how else to describe it. I think I'll maybe let the wet side dry out a little more and like you say, use fresh water rather than the water from the dish. Perhaps as there is no fresh water going into the soil it's become kind of overloaded.

I'm going to keep an eye on it for a week or so, see if the smell dissipates as the substrate dries out.

I think I just panicked a little! I don't want to poison my little friend.