Support Davey23UK it's normal to be full.
Let me try to clarify - when the first sort operation appear, it creates temp segment into the temp tablespace. The temp segment gets extents when needed to increase, but when the sort operation finishes, the extents do not gets released to the tablespace, since there are no other segments in it and so, no one could possibly need this space.
To summarize, there usually is enough free space but INTO the temp segment. The tablespace seems to be full, since most of the space is occupied by the temp segment however.
look at the v$tempseg_usage and V$temp_extent_map as IXION said, to find out how much space is really used