my megane 225 key fob going through batterys?

Hello, I hope that some one can help me, my key fob works to start the car fine but only when slotted in (it does not detect it when just in my bag) I can live with that however the fob card then stopped locking and opening the doors. I called some one out to replace the fob who charged £110....worked for about a week and then stopped again, this started working again when I changed the battery but this is costing me a small fortune as it needs changing every couple of weeks. Does anybody know what is the problem or what I can do to fix this ? Thank you