I think that this is really good Simon. I love the rock texture, your outdoors bit looks great and overall the colours are wonderful. I have some ideas that you are free to ignore:

- the boundary between cave floor and grass is not great. You might consider reworking this completely. For example you could keep a very thin arch of bedrock (narrowest in the middle creating the illusion of an arch) over the cave mouth and extending the grass along the interior edges of the cavern tunnel a little (this can be done easily with a layer mask).
- look at the light/shadows bouncing off your stalagmites and compare them with the actual light in the cavern - they don't match. You can fix this either by rotating each one to fit, or else (better) adding an overlay layers with a layer mask that only includes the stalagmites, then painting black and white on it with a large fuzzy brush to match where the shadows should go, then duplicating the overlay layer.