What is the scratching noise I can hear coming from the wall/floor?

Last night I heard a very loud scratching/tearing noise coming from behind my bed. I know there is a gap between the floor boards and skirting on the external wall which I sealed with a piece of MDF when I moved in but the scratching was extremely persistent and didn't seem to be frightened off by any noise I made. It sounded like something was gnawing at the carpet. I live on the first floor of a converted Victorian house so I am surprised if a mouse is living in the gap between my floor and the ceiling below as I can't see how it could get in there and how it could survive. The wall where the gap is is solid brick so I can't see how a mouse could get in there either. There is no evidence of anything having been in my kitchen and I clean my flat regularly and thoroughly. Also the weather has been very warm recently; I thought mice came inside if it was wet or cold. This is the first time I have heard this noise in 18 months of living in my flat. I would be grateful for any feasible explanation!