My tom 300 wont switch on

SOURCE: tomtom300 gps mains charging +

1= My battery is flat, but when i put it on charge in the house it doesnt charge and no green light comes on, on my sat nav why would this be?

Could be your house charger is bad or the power connector on the back of the Tomtom is bad.

2= But when i put it in car charge it works perfect 'Y'?

You are using the "power shoe" to charge the battery. For a test unplug the power cord from the power shoe and hook it to the back of the Tomtom. If this work then the house charger is bad. If this doesn't work then the Tomtom rear power connector is bad

3= If ever i have to change battery how is this done?

You will have to remove the faceplate and disassemble the Tomtom in order to get to the battery. Or you can find a shop that repairs small electronic. Find the battery on Ebay.

SOURCE: when i plug into car ok but when to mains nothing

First , let me say that the batteries do not fail very often. Answers here to your 3 questions in order ......
1. Your home mains charger is faulty. 2/ Your car charger is probably OK. 3/ Clip out the screen surround. Remove the 4 special STAR screws that are at the corners of the screen. Remove the main circuit board by taking out 4 smaller star screws and let it hang loose but still connected by its cables. DO NOT DISCONNECT THE BOARD CABLES. Remove the speaker . Under the speaker are 2 more screws holding a plastic bracket that clamps the battery. Remove these 2 screws and the battery clip comes off. Unplug the battery cable from the now loose main board and the battery can be removed. Fit new battery and reassemble in reverse order. NOTE , some tomtom 300's have a protective copper or foil sheild covering the main board. The screws holding the board cannot be seen until the foil is removed. Removing this foil and not repalcing it will render your tomtom more prone to interference although the tomtom will still work without it.