I can not find a battery for my 520 tom tom go sat nav all are 1300 mAh and mine is 1320 mAh

The difference in capacity between 1320mAh and 1300mAh is minor (we are talking about a 1,6% difference, this is below production variance anyway). If the voltage and battery type is the same and the new battery fits inside your device, there will be no problem.

Note, the 1300mAh is the capacity of the battery (how much energy it can store). The main information this value gives is how long a device can work without the battery needing a recharge. You need to be concerned only about the output voltage of the battery (measured in Volts, abbreviated V, and the chemistry type of battery like NiMH, LiIon, LiPo, NiCd). The voltage should match the original so that your GPS device works properly without getting fried or underpowered; the chemistry type must match so the battery and the charging electronics do not get damaged when the external charger is connected.