no, depends on how much memory you have, in the options there is a slider bar to allow you to set the amount of memory you want the maps application to use.
the more you allow, the more data it will save.
eg, if you get all the data for derby, then leave derby, it will still store derby and the new location, as long as there is enough memory. if the memory space runs out, it will override the oldest map data stored in memory.
get a 2gb micro sd card.
free net is free whatever you use it for.
you should adda bundle. i added a 10mb bundle for about £8 a month extra on my bill.
10mb is more than enough for the avergage mobile web user.