What on earth are these?

First there was one, now this morning there are two.  Both about a handspan across - 8" / 20cms or so and a little bit deeper than that.  The grass has been pulled up to make a curved entrance and it's very neatly woven together. 

So what on earth has made them?  No evidence of animal droppings, neither Monty nor Pip were excited about them so I'm assuming no lovely smells there for them either.

Is it some sort of ground nesting bird?  Or a mouse house?  Does anyone know who is going to move in?  I'd like to know before I get a shock on the morning walk!  They are quite small structures, so whatever it is will probably be more worried than us.