Thanks for the reply. Sorry, I don't have a digital camera so a picture will be difficult - I'll try and take one on my phone and see if it's decent enough to make it out.
To answer your questions....he's a cave sleeper, the substrate is sand but I've never seen him burrow in it. He's been in the tank for about 6 months and doesn't seem to have grown much. As I said, looks exactly like a yellow except for the colour.