Remote locking and unlocking sensors

[LuS1fer]

I have an 07 Fiesta St and the remote is increasingly unresponsive. Changing the battery helped slightly but the more the car seems to sit round, the less responsive the remote unlocking gets. it can be really frustrating when the car won't unlock.
Unfortunately, it only came with one remote key so I can't even try a spare to see if it is the key.

So there must be a sensor on the car. if so, where is it and can it be replaced and would it make any difference.